4.0 Objectives

In this lab you will learn to:

1. Georeference an image;
2. Create a geodatabase, associated domains and feature classes to store digitized features of a geologic map;
3. Use the "Feature to Line" and "Append" tools to add lines to a feature class;
4. Digitize in heads-up mode, construct a topology, and edit features.

4.1 The Problem

Three of our next four labs focus on the geography and geology of the Mason Mountain Wildlife Management Area, our weekend field trip site for collecting field observations. The Mason Mountain Wildlife Management Area (hereafter WMA or MMWMA), administered by the Texas Parks and Wildlife Department, is a state-owned, ~5300 acre former ranch principally dedicated to studying animal husbandry of "super exotic" African ungulates (Oryx, Kudu, Water Buck, etc.) in a Texas Hill Country habitat.  Research into the ecology of the "central Texas Mineral Region" (a.k.a. Llano Uplift) and applications to wildlife management practices are also conducted there.  Access is restricted; entry is by permission only.  Geologically, the WMA is situated near the boundary of the western Llano uplift (Figure 1), where erosion of Cretaceous carbonates (green in Fig. 1) of the Edwards Plateau has exposed older Paleozoic and Precambrian rocks (blue and pinks in Fig. 1) of the Llano Uplift beneath.  The wide variety of rocks exposed here and extensive outcrops of the Precambrian granite of the Katemcy Pluton (Fig. 2) make this a nice location for studying the geologic history of the uplift and the products and processes associated with magmatic intrusions. 

Figure 1. Generalized geologic map of the Llano Uplift region, showing the location of the Mason Mountain Wildlife Management Area in Mason County.

Figure 2.  Generalized geologic map (data from Geologic Atlas of Texas, Llano Sheet) of the Mason, TX area, including the WMA.  We will not be camping in the location shown on this map.

Our first task, to be completed in this and the following lab (Labs 4 and 5), is to create a vector geologic map of the WMA using a few provided shapefiles and lines that you will digitize from an image of an unpublished UT senior thesis map.  Once completed, this vector map can be used as a base layer for collecting and editing geologic during our field trip.  The geologic map is most useful when displayed with other base map layers (roads, contours, creeks, lakes, etc.) that give it context.  Base map feature classes, to be organized and symbolized, are also provided for this purpose.  In Lab 7 we will generate a highly detailed shaded elevation layer from airborne LiDAR elevation measurements that can also be displayed with these vector base map layers.

4.2 Data

Data (and metadata) for this lab are found in the Lab_4_and_5_data folder in the network class folder. They include:

• A jpeg image of an unpublished 1:7500 scale geologic map of the Mason Mountain Wildlife Management Area.  The image does not have an accompanying world file.  You will create one by georeferencing.
• Infrastructure shapefiles:
  • WMA Roads_2017 - gravel and 4-wheel drive roads within the WMA
  • Buildings2017 - WMA building polygons
  • Fences_2017 - Polyline file of fences within the WMA
  • Gates - Point file of fence gates
  • Property_footprint - Property boundary polygon
• Hydrography and elevation shapefiles:
  • Lakes_2019 - Polygon file of lakes, ponds and tanks
  • Creeks_2019 - Polylines of ephemeral creeks and drainages
  • Contours_20ft - Twenty-foot elevation contours derived from 2007 LiDAR LAS files (more on these in Lab 7)
• Geology Shapefiles (within "Geology_base_layers" folder):
  • K_unconformity_2017 - Polyline of the Cretaceous/Precambrian nonconformity within the WMA
  • Granite_outcrops_2017_updated_all_data - Outcrop exposures of granite (polygons)
  • All_pts_post_2017_trip - Measured orientations of joints, dikes and veins within granite (points).  Also includes points of interest.
  • DK_measurements - Strike and dip of bedding in sandstone or foliation in marble (points)
  • All_pCtm_line_features_post_2017 - linear features in granite, i.e. faults, dikes long enough to map, contacts between granite bodies of different textures.

To complete this lab and Lab 5 you will do the following (in order):

1. Georeference and rectify the geologic map image

2. Create a Personal Geodatabase

3. Import the WMA shapefiles into the Geodatabase

4. Create a Geology Feature Dataset with a spatial domain that encompasses the area of interest and import the Geology shapefiles

5. Create an empty line feature class within the Feature Dataset with the following fields and domain values:

• a) Exposure (exposed, inferred, covered)
  b) Type (fault, contact)
  c) Downside (down-thrown side of fault - N, NE, E, etc.)
  

1. Creates domains for each field and attach the domains to the feature class

2. Create a line map boundary from the map area polygon using the Feature to Line tool; Append this feature to the line feature class

3. Append the provided digitized unconformity to the line feature class

4. Digitize geologic unit contacts and faults

5. Create a contact and fault line topology (END OF LAB 4)

6. Clean the line  feature class of topological errors (BEGINNING OF LAB 5)

7. Create attribute and symbolize geology polygons

8. Add and symbolize base layer feature classes (roads, contours, streams, lakes, etc.)

9. Add and symbolize geology feature classes (outcrop polygons and strike/dip layers)

10. Label geology polygons, faults and point features

11. Layout and print a map

12. Answer and turn in any questions and your layout.

 4.3 Procedure

 4.31 Georeferencing 

1. Copy the Lab_4_and_5_data folder to your network storage.

2. Within your Lab_4_and_5_data folder, create a new folder called My_Data.

3. Open ArcMap with a new, empty document.

4. Add the "property_footprint" shapefile to the map.  The spatial reference for this file is UTM14N, NAD83.

5. Add the geologic map image, "WMA_geomap_2017.jpg".  As indicated by a warning window, this image does not have a spatial reference; the spatial reference is said to be "undefined".

6. Georeference the geologic map image to the "property_footprint" shapefile, which shows precisely the same WMA boundary as that contained within the image. Figure 1 below shows suggested link points for georeferencing.

Consult the lecture notes and the Georeferencing Software Tip for details.  For further details on georeferencing, see pp. 317-322 in the digital book "Using ArcMap" in the class folder (\Digital_Books\ArcMap\Using_ArcMap.pdf).  Also Search ArcGIS Help for "Georeferencing a raster dataset" and "Fundamentals for georeferencing a raster dataset".

Watch a short video of part of the georeferencing process (with a different geologic map).

7. Save your georeferencing links from the Links table in your Lab_4_and_5_data/My_Data folder.

8. Rectify the georefenced map, using nearest neighbor resampling, and a cell size of 1 meter, Tif format and jpg compression of 90.  Before rectifying, be sure the file will be saved in your My_Data folder for this lab.

9. Because the spatial reference of the Data Frame is UTM14N NAD83, the rectified map should also be in this coordinate system.  Check the spatial reference of the rectified map file before proceeding.   Do this in Arc Catalog (within ArcMap) by right-clicking on the new file and examining the file's Properties.  Forget how?  Consult section 2.461 in Lab 2.

10. Add the newly georeferenced map to ArcMap and remove the original image.  Save the ArcMap project to your My_data folder.

Figure 1. Unreferenced geologic map image with suggested points for georeferencing shown by the bright green circles

 4.32 Creating a File Geodatabase and Importing Data Files

1. Within ArcCatalog, browse to your My_Data folder, right-click on the folder, select "New", then "File Geodatabase".

2. Name the new Geodatabase "WMA_Map_XXX.mdb" where XXX are your initials.

3. Right-click on the WMA_Map_XXX.mdb icon, select "Import", then "Feature class (multiple)...".

4. Before importing any  data, we'll first set some "Environment" variables.  This will save some browsing/typing later.  Click the "Environments..." button at the bottom of the window, select "Workspace", click the folder button next to "Current Workspace", browse to your Lab_4_and_5_data folder and click "Add".  This is the only Environmental variable we'll change, so click OK.

5. Time to Import some files...  Using the folder icon next to the "Input Features" line, browse to your Lab_4_and_5_data folder, hold down the Shift key, and click on the shapefiles you wish to import, i.e. all of the shapefiles within your Lab_4_and_5_data folder, excluding those within the "Geology_base_layers" folder.  Click OK and wait for the files to be imported.

6. Examine the WMA_Map_XXX geodatabase in ArcCatalog.  If the above steps were completed correctly you will see 9 feature classes within the geodatabase, as shown below (but the Lakes and Creeks feature classes are now "2019").

Figure 2. ArcCatalog view of successfully imported feature classes within the WMA_map Geodatabase.

 4.33 Creating a Feature Dataset and Importing Geology Feature Classes

We will need a Feature Dataset (see the lecture notes from week 3) within the geodatabase to hold files we will create by digitizing.  Why?   Without a Feature Dataset, the files we will create could not share a topology.  This is a general rules... all files that share a topology must be contained within the same Feature Dataset.  For this reason, all files within a Feature Dataset must have the same spatial reference and "spatial domain" (more on this below).

1. Right-click on your WMA_Map_XXX geodatabase, select "New", then "Feature Dataset".

2. Name the new Feature Dataset "Geology" and click the "Next" button to bring up the now familiar Spatial Reference Properties window.

3. Browse to Projected Coordinate System>UTM>NAD83>NAD83 UTM Zone 14N.prj and select (make sure the right Projected Coordinate System is in "Name"), then click "Next".

4. In the next window you are given the chance to specify a vertical datum.  The default is none, which means that if you have elevation information (e.g. features classes of the type "PointZ", "PolylineZ") that were collected with a particular elevation datum (e.g. often NAVD88 for data collected by most GPS units) the software will not provide a means for converting the data to a different vertical datum.  If you knew the vertical datum for the data sets you were incorporating this would be the opportunity to specify it.  For the purpose of this lab the default of "none" is acceptable.

5. The final window sets the "XY tolerance" (minimum distance between between nodes or vertices before they are considered coincident).  See "About XY Tolerance" in ArcGIS Help.  Accept the defaults and click "Finish".

6. As above (step 5 in 4.32), Import shapefiles, this time from the Geology_base_layers folder (5 in all), into the new Geology feature dataset.

7. As these (or any other) new feature classes are imported (or newly created) in a Feature Dataset they may be (?) automatically added to the ArcMap Table of Contents (T.O.C.).  Remove these so that all that remains in the T.O.C. are the georeferenced geologic map and property_footprint files.

 4.34 Creating New Feature Classes within the Feature Dataset

We now need to create empty feature classes within the Feature Dataset to hold any lines, points and polygons that we will digitize from the georeferenced geologic map, as well as their attributes.  One strategy for doing so is given below.  It is not the only way this could be done, but is relatively simple and straightforward for this fairly simple map.  A more complicated map with more features might require a different scheme with additional feature classes and domains.

Once the feature classes are created, we will "edit" them to store the map features we create by digitizing.  Read about the general process and strategies behind editing by searching ArcGIS Help for "What is Editing?"

1. In ArcCatalog, right-click on the Geology Feature Dataset, select "New", then "Feature Class..."

2. We need a feature classes for geologic lines, i.e. faults and other rock unit contacts.  Name this feature class "GeoLines_XXX" (XXX is again your initials) with an Alias of "Contacts and Faults".  We could separate these into two different line feature classes but for now faults and contacts will be stored in a single feature class.  Change the feature "Type" to "Line Feature". The "Geometry Properties" options here should be left unchecked, as shown below (left).

3. Click the Field Name "SHAPE" to see the "Field Properties" for the Shape field, (below right).

Figure 3. Creating a new line feature class for contacts and faults.

4. The "Field Properties" for the "SHAPE" field of the attribute table for this new Feature Class (which you've named (GeoLines_XXX) are listed in rows (see above right). The SHAPE field will store the "Geometry Type" (in this case lines), coordinates, spatial reference, and other variables of this feature class. For more on SHAPE field property variables see pages 45-48 in the digital book "Building a Geodatabase" or the Help files.

5. We will now add a few new fields to the attribute table.   Enter the field name "Exposure" in the blank row below the SHAPE field name.  For future reference, Field Names can not exceed 13 characters and can't include any special characters, including spaces.  An "Alias" can be specified for longer names and/or coded field names.  The Data Type for this new field is "text" (see above right) and the Field Properties list should be modified as follows:

   • Length: 10 (8 characters are needed to eventually store the values "exposed", "inferred" or "covered"; 10 is overkill).  See the figure above, right.

6. Repeat this process for two new text fields:

   • Field Name: "Line_Type";  Data Type: text, Length: 10 (will eventually have values of "fault" or "contact")

   • Field Name: "Downside"; Data Type: text, Length:3 (this will have values of N, NE, E, SE, S, SW, W, NW, or N to indicate the down-thrown side of the fault)
       

7. Click Finish.  You have now created a feature class that will store geological lines - contacts and faults - with attributes to indicate which one of the two a line represents and whether the feature is exposed, inferred or covered, which will enventually be symbolized by solid, dashed or dotted lines.

Geologic maps, like the one we are digitizing, contain many other features besides contacts and faults.  For example, point features that record orientations of layering and linear features (e.g. dikes) and polygons that represent rock units are nearly always present.  We will have the  software create rock units from the lines that we digitize and, in this instance, you will not digitize the point features.  In a later lab, we will create a features class(es) for point data (strike & dip or trend plunge of joints, lineations, etc.) that we will collect in the field but for now these are provided. 

4.35 Adding Domains to the Geodatabase

To avoid entry errors or repeatedly typing the same values when "populating" the attribute table of the GeoLine_XXX feature class we just created, we will now define lists of all possible attribute values for the fields we created, as well as those for a rock unit feature class that the software will create.  Such lists are called "Domains".  Domains are created for the entire geodatabase, not just for a specific feature class or feature dataset, allowing the same domains to be used by any feature class within the geodatabase.  Once created and attached to the feature classes, domain values can be selected during editing from drop-down menus in the attribute tables, a very fast and efficient way to enter data.

1. In ArcCatalog, right-click the WMA_map_XXX geodatabase icon, select "Properties..." and click the "Domains" tab.

2. The domains you will create have the following names, properties and values:

All of these domains will be applied to text fields, and all will be "Coded Value" domains, storing values as codes.  The codes are a way to speed up searching and sorting of the final tables and have the advantage of providing drop-down menus for data entry.   But using a code (the actual value being encoded) different than the "Description"  produces problems when exporting the data to other applications (Excel, etc.).  I therefore recommend that where practical the values entered for the Code and the Description be identical or nearly so, even though this would seemingly defeat the main purpose of using codes (note that the codes and descriptions given above for Unit_Name domain violate this).  It's won't affect searching or sorting for the small tables that we'll create in this instance, and we won't be exporting data in any event.  Just a word to the wise for later work.  It should also be noted that domain codes are case sensitive - be consistent and either capitalize (as above) or don't capitalize the first letter of all of your coded values.  So that we can all share data after it has been collected in the field, please use precisely the codes given above, paying attention to capitalization of all codes.

3. As shown below in Figure 4, enter each of the above Domain names into a row below the "Domain Name" heading. Leave the adjacent "Description " column blank or type in a description of what the domain name means.

4. Change the first two rows of the "Domain Properties" for each domain to "Text" and "Coded Values", respectively.

5. In the "Coded Values" area, enter the Coded Values for each domain from the Table 1 above, using the exact same code and description for each value, except for Unit_name, where the two are different.  An example for the "Unit-name" Domain is shown below. The "Unit_name" and "Unit_Abbrev" domains will be used for rock unit polygons that we will later (Lab 5) make with ArcToolbox from the lines we digitize. Note in the example below that the Coded Values have underscores and/or dashes, not spaces.  "Codes" can not contain special characters or spaces, but "Descriptions" can.

Figure 4. The database Properties/Domains window, showing domain names and descriptions, with field type, domain type and coded values/descriptions for the Unit_name domain.

6. Click OK.  Other domains and coded values can be added later, if need be.

4.36 Attaching Domains to Feature Classes

The feature class fields we created earlier do not yet have their associated domains.  It would seem more logical to create the domains before creating the feature class so that the domains could be assigned at the same time that the feature classes were created.  This is indeed the recommended procedure... if you have a well thought-out, preconceived database schema!  I usually do it the way I'm describing here...

1. In ArcCatalog, right-click on the GeoLines_XXX feature class in the geodatabase, select "Properties..." and click the "Fields" tab.

2. Click on the Field Name "Exposure".

3. In the "Field Properties" area, click the blank cell to the right of the word "Domain" to reveal a drop-down menu; select the "Exposure" domain.

4. In the blank area to the right of "Default Value", type "Exposed" (note that these values are case-sensitive and must match the case of the values in your domains).  Solid lines are by far the most common type of lines on the geologic map, thus "Exposed" is a good default value for the Exposure field.

5. Notice that the software has automatically added a new field to this feature class: "SHAPE_length", which will be populated by the software as we draw lines.

6. Repeat steps 2-5, for "Line_type" and "Downside" using the appropriate domains from the domain values you earlier entered.

Congratulations, you've now completed the geodatabase needed for digitizing and creating the map for this lab! 

4.37 Digitizing Features

Some general strategies for digitizing, otherwise known as "Editing":

• Digitize a map boundary line first (think of this as a "contact" with the rest of the world). Alternatively, it can quickly be created from a bounding polygon automatically, as we will do below, using the "Feature to Line" tool in the Toolbox.
• Set Snapping before starting and check and/or reset Snapping as new feature classes are digitized (more about Snapping below). Snapping is ABSOLUTELY ESSENTIAL for results that won't require a lot of further editing.
• Try hard to assure that all line features that intersect other lines are snapped to those lines or polygons.  Lines can not cross; a vertex must exist at every line intersection.
• Work from one edge of the map to the other; examine the map carefully and try to think a few steps ahead.
• Enter attributes as you go.  Keep the feature class' attribute entry window, accessible on the editing toolbar, open as you work and fill in the fields after completing each feature.
• SAVE YOUR EDITS OFTEN.  SAVING EDITS IS DONE FROM THE EDITING TOOLBAR, NOT FROM THE ARCMAP FILE MENU.  The editing process can crash the software more easily than almost any other ArcMap process.

  A. Background Info for Digitizing - General Work Flow For An Editing Session 

1. Open ArcMap, add a rectified geologic map image and any other layers useful for digitizing.

2. Check the Coordinate System of the Data Frame.  Set it to match that of the feature classes you will edit.

3. If not already open, open the Editor toolbar (Customize>Toolbars).

4. The generalized digitizing/editing procedure (for future reference) is:

1. From the Editor toolbar menu (or from a right-click on the layer of interest in the Table of Contents), Start Editing. If the Data Frame coordinate system is different from that of the editable feature classes in your Table of Contents, you will see a warning window similar to this:



Figure 5. Editing warning that the spatial reference of the listed feature classes does not match the spatial reference of the data frame.  This will not prevent editing in this instance, but may in others.  If the feature class being edited is not listed then ignore the warning and proceed.  If it is then consider changing the data frame spatial reference to match the feature class being edited.  Failure to do so may result in data that are not properly located.

2. When/if this window appears and the layer(s) you will edit are listed, "Stop Editing", go to the Data Frame Properties, and change the coordinate system to match that of the feature class(es) you will edit, in this case NAD83 UTM Zone 14N.  Although it may list many layers, the only relevant warnings are for those you will edit.  Ignore any others and hit "Continue" if the layers you will edit are not listed.

3. After this warning window, the "Create Features" window (or Tab) opens, allowing you to select a feature class to edit and to choose a "Construction Tool"

.

Read about the "Create Features" window by searching ArcGIS for "Creating features with feature templates".

4. Open the "Snapping Toolbar"  (under the Editor menu on the Editor Toolbar) . So what is this snapping business about?

 

See "About Snapping" in ArcGIS 10 Help.  READ THIS NOW  - IT IS WELL WORTH THE TIME, CONSIDERING WHAT FOLLOWS.

5. Begin tracing or outlining a feature – create a “Sketch”.  Click to create a vertex; create vertices as needed to outline the feature.  How many vertices and to what detail do you need to digitize?  This question is best answered by the source layer map scale.  It is senseless to digitize at a scale greater than 2X the map scale - the map was likely never intended to have a degree of accuracy that exceeds the map scale, so even 2X is overkill.  In our case, the original map has a scale of 1:7500, so zooming in to more than 1:3750 to digitize is overkill!

6. Finish the feature outline with a double click, OR a right-click, then "Finish Sketch" OR press F2.  If you don't "Finish" before Saving, you will lose your edits.
   

7. SAVE EDITS (on editing toolbar menu, NOT the ArcMap toolbar).
   

8. Open the table or the attribute editing tab for the newly created feature (table icon on edit toolbar) and enter attributes.

9. SAVE EDITS.

10. Repeat for the next feature.

B. "Digitizing" the Map Boundary

The map boundary, in this case the property boundary, is a rock unit contact with the rest of the world so needs to be stored as a line.  Rather than carefully and tediously tracing the property_footprint polygon, it is easier to create an exact replica of the polygon lines using the Toolbox tool "Feature to Line", and then to "Append" the result to your GeoLine_XXX feature class.

1. Use the "Search" tool in the ArcMap menu to find the "Feature to Line" Toolbox tool, open it, "Show Help" and read the Help information for this tool.

2. Use the tool to create a line feature class. Change the Input to property_footprint and accept all of the defaults EXCEPT the "Output Location", which should be the Geology feature dataset in your WMA_map_XXX geodatabase.  Name the new line "WMA_boundary".

3. Use the "Search" tool again to find the "Append" (Data Management) tool, open it, "Show Help" and read the Help information for this tool.

4. Use the tool to append your new boundary line feature class to your GeoLines_XXX feature class.  IMPORTANTLY, set the "Schema Type" to "No Test" (see the tool Help for the reason why) and accept all other defaults.

5. The GeoLine_XXX feature class now has a feature stored (to see it, turn off all of the other layers in TOC), but it doesn't have any attributes.  These can be added by editing the attribute table.  "Start Editing", open the GeoLines_XXX attribute table, and click on one of the empty attribute cells.  You should then see a drop-down menu; choose a proper value from the list, i.e. these are "exposed" "contacts".

6. Save your edits and stop editing.

  C. Digitizing Other Geolines

What remains are other rock unit contacts and faults.  We will not digitize the individual Cretaceous rock units shown in blue and greens in the northern part of the map.  To repeat, we will not digitize the individual Cretaceous rock units shown in blue and greens in the northern part of the map, do not waste time doing so.  Instead, a single contact that encompasses all of the Cretaceous rocks has been provided - the K_unconformity feature class (Yahoo!).  This needs to be "Appended" to the GeoLines_XXX feature class, then attributed, following the procedure of Steps 3-6 above.

1. Append and attribute the the K_uncoformity feature class to the GeoLines_XXX feature class by repeating Steps 3-6 above.  Exposure is "inferred".

2. You have now "digitized" the unconformity between the green and pink rock units on the map. Do not trace it again!

3. If not already in Editing mode, select "Start Editing" from the Editor drop-down menu, or right-click on the GeoLine_XXX feature class in the Table Of Contents and select "Edit Features>Start Editing".  If not already open, click on the "Create Features" tab on the right side of the ArcMap window to open the "Create Features" window. Click "Contacts and Faults" and choose the "Line" tool, as shown below.
                                                                    
   Figure 6.  The Create Features window, open to show the Contacts and Faults feature class and the line tool selected for editing.

4. From the Editor toolbar drop-down menu select "Snapping" to open the Snapping toolbar.  We wish to snap, in this case, to the "edge" of the property boundary lines, not their "vertices" or "ends".  Click the single icon on the Snapping toolbar that will allow you to do so, as shown below with the blue-highlighted icon.  In other instances you may want to snap to vertices or ends - use these options as circumstances dictate.
                                                                   

 


Figure 7. The Snapping toolbar, with the "snap to edge" tool selected.  The other choices are "snap to end", "snap to vertex", or "snap to point" feature.  More than one can be active at a time.

Snapping is absolutely essential when digitizing.  Snapping is absolutely essential when digitizing. Snapping is absolutely essential when digitizing... It is impossible to guess when a line you are digitizing is touching another line unless you snap to it.  Gaps between lines ("undershoots") or "overshoots" can be fixed after a topology is created, but it is much easier to get it right the first time!

5. Set the map scale to 1:7,000.  This is sufficient detail, keeping in mind that the original map scale is 1:7500.  DO NOT WASTE TIME AT A LARGER SCALE ADDING MORE VERTICES THAN ARE WARRANTED OR NECESSARY.

6. Work your way around the map, beginning lines by snapping to map boundary lines when you can and carefully tracing lines elsewhere. Double-click to finish, or right-click and "Finish Sketch", or use the F2 key after each feature is completed.

7. Remember these rule:

   • Lines should not be duplicated. They must either start and end at other lines, or close on themselves to become "islands", not touching any other line.

   • Lines must snap to other line edges or vertices and can not cross.  They can abut one another at a common vertex and continue on, but they can not cross.

8. Click the Attributes button on the Editor toolbar, click a field name (e.g. "Exposure") and then select the proper domain values from the drop-down menu.  An fault entry in progress is shown below (but note we are not storing "Fault Type" here, all faults are normal faults).  Use the georeferenced map to established where a contact or fault is inferred (shown as a dashed line), exposed (solid line) or covered (dotted Iine). Do not edit the OBJECTID or SHAPE_Length attributes.

Figure 7.  Assigning fault attributes with the Attribute editor.

Watch a short video of digitizing faults (from a different map).  Note that the down-side attribute is incorrect for some of the faults.

5. SAVE EDITS.

6. Use the pan and zoom tools to navigate the map, digitizing and attributing faults as you go.  Topology dictates that faults and contacts can not cross one another or each other (geologic reasoning dictates this too!).  End the fault  or contact you are digitizing and start a new one when the fault or contact you are digitizing intersects another. The new contact or fault should begin by snapping to the start, end or edge of one that is already finished.  All faults are presumed to be normal faults, and the down-thrown side will always be the side with the youngest rock unit.  The relative ages of the rock units can be found in the Explanation of the map - they are properly listed from youngest to oldest.

7. To delete a line once it's finished, select it (using the selection tool on the Editor toolbar), right-click and choose Delete .

8. To delete or add a vertex to a completed line, select the line with the Edit (arrowhead) tool on the Editor toolbar, right-click on the line and choose "Edit vertices" (or select the "Edit vertices tool from the Editor toolbar, or even more simply, double-click on the line with the Edit tool), right-click on the vertex you wish to delete and select "Delete Vertex"; to add a vertex, right-click on the line where you want to add one, then select "Insert Vertex" .  Vertices can be moved by dragging while in the "Edit vertices" mode.
      Yet another way to do this is with the Edit Vertices toolbar, which is active and on-screen whenever you are in the "Edit vertices" mode:
                                                                                   
                                                                       Figure 8. The Edit Vertices toolbar.

9. To split a line into segments, select the line with the arrowhead tool, find the Split line tool on the Editor toolbar and click where you want to split.

10. SAVE EDITS frequently.  Once they're saved, the program can crash and you won't loose any work.

For more on how to create and modify line features, see "Editing vertices and segments" in the ArcGIS Help.

11. A final word about editing... selecting features for editing can be difficult if more than one layer is selectable - you can accidentally select a layer that is underneath the one you're trying to select.  To avoid this problem, the "selectability" of layers can be turned on or off.  The easiest way to do this is by changing the TOC view to show layer selectability, as discussed in the last lab.  Likewise, when you try to select a layer and can't, check the Selection TOC view to see if the layer is turned off for selection.

Watch a short video of digitizing faults and adding/deleting vertices (from a different map). Note that the down-side attribute is incorrect for the first fault digitized.

Watch a short video of digitizing contacts (from a different map).

  Your completed GeoLines_XXX feature class should look the image below.

Figure 9. Completed lines of the GeoLinesXXX feature class.  Faults are in red, contacts in black, either solid or dashed.

4.38 Create a Topology for the Map Lines

Before automatically creating rock unit polygons from the GeoLines_XXX feature class, the lines must be "cleaned" of errors that will corrupt polygon creation -polygons will not be created if there are gaps between bounding lines, and extra polygons can be created when lines overlap.  This is reliably done by creating a topology layer in the Geology feature dataset that contains rules designed to spot errors.  After setting up the rules and creating the topology, the topology can be "validated", and explicit violations of the rules will be flagged for editing.

1. If you haven't already done so, Save your edits and stop editing.

2. From the ArcCatalog window within ArcMap (or from the ArcCatalog program), right-click on the Geology feature dataset in your WMA_map_XXX geodatabase, select "New", then "Topology".  The Topology wizard opens.

3. Click "Next", accept the name the new topology ("Geology_Topology"), and change the cluster tolerance to 0.1 (0.1 meter; see the description of cluster tolerance in the Help files).

4. Click "Next" and place a check in the boxes adjacent to the "GeoLines_XXX" feature class - this is the only feature classes we are checking for dangling and/or overlapping lines.

5. Click "Next" and change the "Number of Ranks" to 1.

6. Click "Next" to bring up the Topology Rules dialog.  We want to know where contact lines dangle (not meeting other contact lines), where they cross other lines and where they cross themselves.
    a) Click the "Add Rule..." button and select the rule (from the drop-down menu) "Must Not Overlap".
    b) Repeat step a), this time choosing "Must Not Have Dangles".
    c) Repeat step a), this time choosing "Must Not Self-Intersect".
    d) Repeat step a), this time choosing "Must not Self-Overlap"
   

For a nice explanation of all available topology rules, search "Geodatabase topology rules and topology error fixes" in ArcGIS Help.

6. Click "Next" and review a summary of Topology properties.

7. Click "Finish" and wait for the Geology_Topology feature class to be created.  Answer "Yes" to Validate the topology now.

8. A new feature class has been created in the Geology feature dataset that highlights every rule violation.  In general, some of these may be valid exceptions to rules, others are errors.  To see the violations, preview the topology feature class in ArcCatalog by right-clicking on the new file, selecting "Item Description" and then the "Preview" tab.  The pink squares and lines are the locations of errors, which we will later view on top of contacts feature class in ArcMap.  To get a list of errors, right-click on the topology layer, select "Properties", click the "Error" tab and click the "Generate Summary" button.  You can also view the errors in ArcMap by adding Geology_Topology to the ArcMap Table of Contents, as is shown in the image below.

If you've done a careful(?) job of digitizing and snapping, your Summary might look something like the one shown below.  The Summary shows 29 errors for the "Must not overlap" rule and a few for self-overlap and self-intersect.  The cluster tolerance error indicates a feature smaller than the 5 meters, probably an extraneous point from an invalid line. Yours may be better (wouldn't that be great!) or worse (ugh).

Figure 10. GeoLinesXXX feature class with 34 topology errors, highlighted in pink, as summarized in the topology error table.

End of Digitizing Part 1 (Lab 4)

In Part II of this Lab (Lab 5, next week) we will go through and fix the errors before going on to make rock unit polygons, attribute them, and complete the map.