N.B. This lab was updated on 9-13-11 for ArcGIS v. 10.0.  An older version for ArcGIS 9.2& 9.3 software can be found here.  An even older version of this lab for ArcGIS  9.1 can be found here.

 4.0 Objectives

In this lab you will learn to:

1. Georeference an image

2. Create geospatial data and store it in a Geodatabase

3. Clip feature classes to a bounding box and join tables

4. Digitize in heads-up mode, construct a topology, and edit features

4.1 Data

Data for this lab are found in the Lab_4_data folder in the network class folder drive. They include:

• DOQQs (6, 50 cm resolution orthophotos: Art NW, NE, SE, SW and Castell NW, NE) from TNRIS. These are jpeg2 format.
• Tiff image of geologic map (gat_sheet_castell.tif) created from a partial scan of the 1:250,000 Llano Sheet of the Bureau of Economic Geology, Geologic Atlas of Texas.  A scanned version of the map with zoom capabilities and all collar information, including an explanation of rock units, is also online courtesy of the Texas Water Development Board.  A version with all collar information can be downloaded from TNRIS at this link.  A paper copy with collar information and a legend can also be checked out from the Walter Geology Library.
• Roads - shapefiles for Llano and Mason Counties, and a lookup table that contains descriptions for the road "levels" codes.  Data were created from TXDOT county map Microstation Drawing Files, online at TNRIS.
• Texas counties shapefile - from ESRI
• Hydrology - a shapefile ( NHD_streams_Llano.shp) of streams created from the National Hydrography Dataset
• Hypsography - a shapefile of vector contours, from TNRIS
• A comma delimited text file, TXunits.csv, from the Online Spatial Data library of the USGS containing information about the rock units on the geologic map.

4.2 Tasks

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

1. Georeference and rectify the geologic map image

2. Create a Personal Geodatabase

3. Import the contours, roads, county line and streams shapefiles into the Geodatabase

4. Create a Feature Dataset with a spatial domain that encompasses the area of interest

5. Create empty feature classes within the Feature Dataset for:

• a) map boundary (polygon; will allow snapping)
  b) geologic contacts (lines)
  c) faults (lines)
  d) granite dikes and marble lenses (lines)
  e) towns, windmills, and ranches (a point feature class)

6. Creates attribute fields and domains for each of the feature classes

7. Digitize a map boundary polygon

8. Digitize and attribute faults

9. Digitize and attribute point features (towns, windmills, ranches)

10. Digitize geologic unit contacts

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

12. Clean the faults and contact feature classes of topological errors (BEGINNING OF LAB 5)

13. Create and attribute geology polygons

14. Clip the roads, contours and streams feature classes to the map boundary

15. Join the lookup table of road levels to the roads feature class

16. Import a roads layer file and symbolize the roads

17. Symbolize and label the streams and contours feature classes

18. Symbolize and label geology polygons, faults and point features

19. Layout and print a map showing roads, contours, streams, towns, ranches, windmills, faults and geological units

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

 4.3 Procedure

 4.31 Georeferencing 

1. Copy the Lab_4_data folder to your network storage.

2. Within your Lab_4_data folder, create a new folder called My_Data.

3. Open ArcMap with a new, empty document. 

4. Load the 6 orthophotos from the DOQQ folder.  The spatial reference for the photos is UTM14N, NAD83.

5. Load the scanned geologic map, gat_sheet_castell.tif.

6. Georeference the geologic map.  The image 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.

7. Save your georeferencing links from the Links table in your Lab4_data/My_Data folder.

8. Rectify the georefenced map, using nearest neighbor resampling, and a cell size of 10 meters.  Before rectifying, be sure the file will be saved in your My_Data folder.

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 by right-clicking on the new file and examining the file's Properties.  Forget how?  Consult section 2.461 in Lab 2.

 4.32 Creating a Personal Geodatabase and Importing Data Files

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

2. Name the new Geodatabase "Castell_Map.mdb"

3. Right-click on the Castell_Map Geodatabase 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 "Evironments..." button at the bottom of the window, select "Workspace", click the folder button next to "Current Workspace", browse to your Lab_4_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_data folder, hold down the Shift key, and click on the shapefiles you wish to import, i.e. both roads shapefiles, the contour shapefile, the Texas counties shapefile, and the streams shapefile.  Click OK and wait for the files to be imported.

6. Import the Roads lookup table (lookup_code2.dbf) using the same technique but using right-click "Import">"Table (single...)".

7. Examine the Castell_Map geodatabase in ArcCatalog.  If the above steps were completed correctly you will see 5 feature classes and a table within the geodatabase, as shown below.

 4.33 Creating a Feature Dataset

We will need a Feature Dataset (see the lecture notes from last week) 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), which we will establish when the Feature Dataset is created.  The procedure is somewhat different for versions 9.1 and 9.3 of ArcCatalog; directions below pertain to version 10.0.

1. Right-click on your Castell_Map 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 "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).  For more on this topic, click the "About Setting Tolerance" button.  Accept the defaults and click "Finish".

 4.34 Creating Feature Classes within the Feature Dataset

We now need to create empty feature classes within the Feature Dataset to hold the lines, points and polygons we will digitize, as well as their attributes.  Given below is one strategy for storing the features of this geologic map.  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.  Read about the general process and strategies behind editing by searching ArcGIS Help for "What is Editing?"

1. Right-click on the Geology Feature Dataset, select "New", then "Feature Class..."

2. Name the feature class "Map_Area" then select a "Type" for the drop down menu, in this case the default (Polygon Features); select "Next".

3. Click the Field Name "SHAPE" to see the "Field Properties" for the Shape field, as shown below.

4. The "Field Properties" for the "SHAPE" field of the attribute table for this new Feature Class (which you've named "Map_Area") are listed in rows. The SHAPE field will store the "Geometry Type" (in this case a Polygon representing the footprint of the map area), 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. We don't need to change anything, nor do we need to add any fields to the attribute table for this feature class.

5. Click "Finish". You have now created a polygon feature class that will hold your digitized outline of the map area. This polygon will be very useful as a bounding box to "snap" the ends of lines to as you digitize.

We now need two new feature classes for lines: one for faults and one for unit contacts.  These could be contained with a single feature class (both are lines), but we will find it useful to keep them separate.  Faults can be unit contacts but may also exist within units and "dangle", extending beyond where two units meet.

6. Repeat steps 1-3 above, using the name "Faults" and changing the "Type" to line, then click "Next".

7. We will now add a few new fields to the attribute table.   Enter the field name "Type" (for fault type) 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" and the Field Properties list should be modified as follows:
   
        Alias: Fault type
        Length: 12 (11 characters are needed to eventually store the values "normal", "reverse" or "strike-slip"; 12 is overkill).  See the figure below.

8. Repeat this process for two new text fields:
      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)
      Field Name: Exposure     Data Type: text
        Length: 10 (a field for values of Exposed, Covered, Inferred, corresponding to solid, dotted or dashed lines on the original map)

9. Click Finish.
   Create a new line Feature Class for un-faulted contacts, named "Contacts" following steps 1-3 and 6 above, and using the procedure in step 7, create a new field for:
     Field Name: Exposure     Data Type: text
         Length: 10 (a field for values of Exposed, Covered, Inferred, corresponding to solid, dotted or dashed lines on the original map)

10. Click Finish.
    

11. Create a new line Feature Class for dikes, named "Dikes" following steps 1-3 and 6 above, and using the procedure in step 7, create a new field for:
      Field Name: Exposure     Data Type: text
          Length: 10 (a field for values of Exposed, Covered, Inferred, corresponding to solid, dotted or dashed lines on the original map)
     Field Name: Rock_type    Data Type: text
         Length: 25 (values are aplite, pegmatite, marble lens)
    (Note that at the scale of this map dikes (and marble lenses) are represented as line.  On a larger scale map these might well be polygons.)
    

12. Create a new point Feature Class for point features, naming it "Map_Points" with an Alias of "Geographic Points".  

13. In the drop-down menu for the SHAPE field property, change the Geometry Type to "Point".

14. As in step 8, Add the following Fields:
      Field Name: Type        Data Type: text
        Alias: Feature Type
        Length: 25
      Field Name: Name      Data Type: text
       Alias: Feature Name
       Length: 50

15. Click Finish.

4.35 Adding Domains to the Geodatabase

To avoid entry errors or repeatedly typing the same values when "populating" the attribute tables of the feature classes we just created, we will now define lists of all possible attribute values for most of the fields we created.  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 from drop-down menus in the cells of the attribute tables, a very fast and efficient way to enter data.

1. Right-click the Castell_map geodatabase icon, select "Properties..." and click the "Domains" tab.

2. The domains to be created 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 different than the "Description" produces problems when exporting the data to ArcPad and other applications, as might be desired if digitizing were to be done in the field.  I therefore recommend that the values entered for the Code and the Description be identical, even though this would seemingly defeat the main purpose of using codes.  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.

3. 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 above table, using the exact same code and description for each value.  An example for the Fault_Type Domain is shown below. The Unit_name and Unit_Abbrev domains will be used for rock unit polygons that we will later (Lab 6) make with ArcCatalog from the lines we digitize. Note in the example figure below that the Coded Value for Strike-slip is entered with an underscore, not a dash.  Coded values can not contain special characters or spaces.

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

4.36 Attaching Domains to Feature Classes

The feature classes we earlier created do not yet have associated domains.  It would seem more logical to create the domains before creating the feature classes 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!  Mine rarely are, so I usually do it the way I'm describing here.

1. Right-click on the Contact 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.  Solid lines are by far the most common type of lines on the geologic map, thus "Exposed" is a good default value for the Contact Type 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. Click OK.

7. Repeat steps 1-6, using the appropriate domains and defaults, for the Faults, Dike and Map_Points feature classes.  Note that a few of the fields (e.g. "Name") do not have domains.

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 polygon first (e.g. the Map_Area feature class).  This polygon can be used for clipping other feature classes to the map area and provides a snapping target for contacts and faults.
• With snapping turned on, digitize the same boundary with four lines saved to "Contacts" feature class.  These lines at the map boundaries are "contacts" with the world outside the map and will be needed to construct a topology.
• 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 a result that will  not later 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.
• Attribute 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. Digitizing the Map_Area Polygon

1. If not already open, open ArcMap and load the rectified geologic map and the five feature classes you just finished creating (Contacts, Faults, Dikes, Map_Area and Map_Points).

2. Check the Coordinate System of the Data Frame.  Set it to match that of the feature classes you will edit; in this case it should be set to NAD83, UTM Zone 14N. 

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

4. The generalized Digitizing/Editing procedure is:

1. From the Editor toolbar menu, 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 the following window:



2. When/If this window appears, "Stop Editing", go to the Data Frame Properties, and change the coordinate system to match that of the feature class you will edit, in this case NAD83 UTM Zone 14N.
   
   
3. When the Create Features window opens, select a feature class to edit and 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.
   
   
6. Finish the feature outline with double click, or a right-click, then "Finish Sketch";
   
   
7. SAVE EDITS (on editing toolbar menu, NOT the ArcMap toolbar).
   
   
8. Open the table for the newly created feature (table icon on edit toolbar) and enter attributes.
   
9. SAVE EDITS.
   
   
10. Repeat for the next feature.
    
    
5. To digitize a rectangle:
   
   
   1. Start Editing;
      
      
   2. In the "Create Feature" window choose Map_Area and the Polygon tool (as shown below);
      
      
   3. Open the Snapping Toolbar;
      
      
   4. Click on one corner of the georeferenced map, move to another corner, then another where you finish with a double click (or follow a single click by pressing the F2 key, or right-click and "Finish Sketch").  Don't be concerned if your polygon does not exactly conform to the edge of the scanned map - try for the closest fit but don't sweat it.

   

   There are no attributes to enter for the rectangle, so you can clear the selection (From the Selection drop-down menu at the top of the screen, click "Clear Selected Feature") and SAVE EDITS from the Editor Toolbar (not the ArcMap File menu).

   5. Change the symbology of the new polygon to "No Fill" with a bright green outline 1 point wide.
      
      
   6. Right-click on the Map_Area layer name in the table of contents (TOC) and select "Zoom to Layer".

  B. Digitizing Faults

1. If not already in Editing mode, select "Start Editing" from the Editor drop-down menu.  In the Create Features window, click "Faults" and choose the "Line" tool.

2. From the Editor drop-down menu select "Snapping" to open the Snapping toolbar.  We wish to snap to the ends or vertices of other faults, and to the edge of the Map Area polygon.  Click the three icons on the Snapping toolbar that will allow you to do so, as shown below with the blue-highlighted icons.
                                                                   

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!

3. Work from the upper left to lower right across the map to digitize the faults.  Begin by zooming into faults that intersect the left edge of the map (a scale of about 1:30,000 works well).  Double-click to finish a line, or right-click and "Finish Sketch" or use the F2 key.  Use vertices sparingly; most faults are relatively straight and don't require more than a few vertices.

4. Click the Attributes button on the Editor toolbar, click a field name (e.g "Downside") and then select the proper value from the domain values given in the drop-down menu.  An entry in progress is shown below.  Do not change the OBJECTID or SHAPE_Length values.

Watch a short video of digitizing faults.  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.  For looping faults that change trend by more than 90 degrees, there can be no one correct Downside attribute.  These should be digitized as two or more separate faults so that each can receive a proper Downside attribute.  Likewise, topology dictates that faults can not cross one another (geologic reasoning dictates this too!).  End the fault you are digitizing and start a new one when the fault you are digitizing intersects another. The new fault should begin by snapping to the end of the fault you just finished.  Finally, all faults are presumed to be normal faults, and the down-thrown side will always be the side with the youngest rock unit.  The ages of the rock units can be found in the TXunits.csv file in the Lab_4_data folder.  They are also in relative order by color in an online version of the Llano Sheet that contains the explanation.  Some of the down-thrown sides of faults are also symbolized with a "ball and bar".  Ask Karen if you need help.

7. To delete a fault 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, new to ArcGIS 10.0, is with the Edit Vertices toolbar, which is active and on-screen whenever you are in the "Edit vertices" mode:

9. 

9. 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.

10. 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. Note that the down-side attribute is incorrect for the first fault digitized.

  C. Digitizing Dikes and a Marble Lens

Four thin granite dikes, symbolized with bright red lines on the scanned map, are present in the NE corner of the map within the pCvs unit near Hwy 29.  Black label lines (these are not faults) connect the letter "o" to these, indicating they are Oatman Granite or, more generically, aplite.  Similarly, a navy blue line amongst the faults near the SW corner of the map has a label line connected to an "m", indicating a marble lens within the pCps unit.   At the scale of the map, these features are best digitized as single lines, not as rock unit polygons.  If you have difficulty identifying these, look at a more legible online version.

1. Following the procedures above, digitize and attribute the granite dikes and marble lens.

2. SAVE EDITS.

3. Open the Attributes window and fill in the attributes.

4. SAVE EDITS.

  D. Digitizing Map Points

Map points are the towns, ranches and windmills on the map.  They are the easiest of all features to digitize, requiring just a single click.  The map shows the towns of Castell and Art, 4 windmills and 2 ranch houses (as small black dots or squares).

1. In the Create Features window, choose Map_Points (depending on whether you gave this layer an Alias e.g. Geographic Points);

2. Click the Point tool, then click on a point feature on the map.

3. Open the Attributes window and fill in the attributes.  If the feature is a town, type in the name (Art or Castell), otherwise fill in the field with Ranch or Windmill.

4. SAVE EDITS.

 E. Digitizing Rock Unit Contacts

Now for the tricky part...
Digitizing the complicated geometry of the rock unit contacts on this map (or any geologic map) requires diligence and attention to detail.  The mechanics of the process are no different than those just completed, however it is easy to forget a few import details:

a) Lines can not be duplicated. They must either start and end at other lines, or close on themselves to become "islands", not touching any other line.
b) 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.
c) Faults and the bounding rectangle are also contacts, the latter with the surrounding world. For fault and map edge contacts to correspond exactly to these features, set snapping to fault line vertices and ends. If you trace the MAP_AREA polygon with four lines that are within the contacts feature class, then you simply have to snap to these lines when a contact reaches the edge of the map. Digitize these four bounding contacts by snapping to the vertices of the MAP_AREA polygon.

Ignore these rules at your peril.  It is very difficult (but not impossible) to construct topologically sound rock unit polygons from digitized contact lines that don't follow these rules.

To simplify the process somewhat, we will not digitize the uncolored outline of the Llano River in the eastern half of the map - simply continue lines across the river as if it weren't there.

1. Open the Snapping toolbar (Editor toolbar, Editor drop-down menu) and turn on Vertex, Edge and End snapping. 

2. If not already editing, select "Start Editing" from the same menu.

3. In the Create Feature window select "Contacts" and choose the "Line" Construction Tool.

4. Set the scale to 1:50,000, right-click on "Layers" in the TOC, and select "Reference Scale" then "Set Reference Scale".  You can, of course, digitize at a larger scale but this greatly exceeds the accuracy of the original map and will cost you a lot more time.  DIGITZE LINES AS THEY APPEAR AT 1:50,000; DO NOT WASTE TIME WITH MORE VERTICES THAN ARE NECESSARY.

5. The first "contacts" to digitize are the contacts of the rock units with the surrounding world, in other words the map boundaries. 

   1. Set snapping to snap only to vertices .

   2. Digitize a line that traces the outline of the Map_Area polygon.

6. To digitize rock unit boundaries, turn snapping for edges and ends back on, begin in the NW corner and work Eastward and Southward, digitizing lines, snapping to the boundary or other lines, and attributing as you go.  Actually, you should not have to do any attributing if the default for "Exposure" is set to "Exposed" - these are all solid lines.  Get comfortable, this will take some time...

7. If you make mistakes, see the procedure above for removing or adding vertices, and for deleting lines.

8. SAVE EDITS often.

Watch a short video of digitizing contacts.

  4.38 Create a Topology for the Map Lines

Before creating rock unit polygons from the Map_Area, Contact and Fault lines, it is useful to "clean" the lines of errors that will corrupt polygon creation.  This is most easily (?) 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 easy editing.

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

2. Click "Next", accept the name the new topology ("Geology_Topology"), and change the cluster tolerance to 15 (15 meters; see the description of cluster tolerance).

3. Click "Next" and place a check in the boxes adjacent to the "Contacts" and "Faults" feature classes - these are the feature classes we are checking for dangling and/or crossing lines.

4. Click "Next" and change the number of ranks to 2.  Change the rank for Contacts to 2; this will allow Contact line nodes to move rather than faults (they should stay relatively straight lines) if snapping is done during the creation of the topology.

5. Click "Next" to bring up the topology rules dialog.  We need rules for both contacts and faults.  As already stated, we want to know where contact lines dangle (not meeting other contact lines) and where they cross themselves.  We'd also like to know the latter for faults.
     a) Click the "Add Rule..." button and, for the Contacts feature class, 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 steps a) and c) for the Faults feature class.  Do not repeat step b) for Faults; faults are allowed to dangle.

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 Topology feature class to be created.  Answer "Yes" to Validate the topology now.

8. A new feature class has been created that contains flags for every rules violation.  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 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.

If you've done a careful job of digitizing and snapping, your Summary might look something like the one shown below.  The Summary shows 3 errors for the "Must not overlap" rule and 5 for the "Must not have dangles".  Yours may be better (wouldn't that be great) or worse (ugh).

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.