Spring 2007
   GEO327G/386G: GIS & GPS Applications in Earth Sciences


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  Messages>Labs>Tips>Tip 4    

4. 

 How to georeference an image

   
 


When loaded into ArcMap, images that contain no spatial reference information are typically placed near the intersection of the equator and Prime Meridian – the x and y coordinates of the data are usually close to x=0, y=0. If not, then the data may actually be stored in geographic or projected coordinates and ArcMap cannot project them on-the-fly because the data’s spatial reference has not been explicitly defined (see the example near the end of Lab 2). This latter problem is easily solved (if the spatial reference is known) by simply defining the spatial reference for the data in ArcCatalog. To do so:

  • Right-click on the file’s icon in ArcCatalog
  • Select Properties>Spatial Reference>Edit and select or enter the proper datum and coordinate system.

When image data are relative to an arbitrary set of x and y axes, or when they are stored in an unknown geographic or projected coordinate system, georeferencing tools in ArcMap can be used to align the data to their real world location. To do so requires either:

  • spatial coordinates (e.g. GPS readings or lat./lons. on the image) for a minimum of three points on the image
  • a minimum of three points on the image that can also be located on spatially referenced data (e.g. georeferenced DOQ or DRG), that are already loaded in ArcMap.

A) To georeference with GPS or survey coordinates:

  1. Open an new ArcMap document.
  2. Check the coordinate system of the Data Frame to establish that it is Undefined. If not, clear it with the Clear button (Right-click on the Data Frame name in the TOC, then Properties>Coordinate System>Clear>Apply>OK).  (Certain problems are created if an image without a spatial reference is loaded into a Data Frame that has a defined coordinate system.)
  3. Add the image to be georeference to the empty ArcMap document.
  4. Turn on the Georeferencing Toolbar (Tools>Customize>check the box next to Georeferencing, then Close)
  5. Open the Links Table by clicking the table icon on the new toolbar.
  6. Zoom into the first GPS control point on the image to be georeference.
  7. Click the Add Control Point tool and with this tool click once on the image control point and again anywhere else in the display.  A set of source and map coordinates will appear in the Links Table.
  8. Replace the map coordinates for this control point with its GPS coordinates.
  9. Repeat this process for at least two other points.
  10. When the Link table is completed, you can Save the control points by clicking the Save button at the bottom of the Link table window. (Should you mess up and later have to relocate control points, you could simply Load these points into a new georeferencing table.)
  11. To write a new file that is spatially referenced (so you’ll never have to do this again for this image…), from the georeferencing toolbar drop-down menu select “Rectify…”, fill in the dialog box (see lecture notes for details), and Apply.
  12. Although the image is now saved with geographically meaningful coordinates, the coordinate system will still be undefined. To define it (e.g. the GPS data were recorded in UTM, DD or some other reference relative to a datum of WGS84, NAD27, etc.), follow the two steps at the top of this tip.  This last step is crucial if the data are to be used with data that are in a different coordinate system.

B) To georeference relative to data that are loaded and properly referenced in ArcMap:

  1. Add the referencing data (e.g. roads, streams, a DOQ or DRG)
  2. Determine the coordinate system of the Data Frame (see step 2 above) – this will be the coordinate system that your new georeferenced file will eventually be stored in. If you wish to change it, do so now.
  3. Add the image that will be georeferenced to ArcMap. It will likely not be visible at this stage, being located either near the intersection of the Prime Meridian and equator or somewhere else far removed from the referencing data.
  4. Zoom to the data that will be used for referencing ( e.g. your vector data or DOQ).
  5. Turn on the Georeferencing Toolbar (GT; see step 4 above).
  6. Make sure the Layer drop-down list on the GT is set to the image you will georeference, then from the Georeferencing drop-down menu select “Fit to Display”. This should place your image in the center of the Data Frame.  If it lies on top of the data you will use for referencing, reorder the layers in the TOC so the image is beneath the other data.  Adjust the transparency of the layers as needed (in the Layer Properties, Display tab).
  7. Open the Links Table by clicking the table icon on the GT.
  8. Zoom into the first control point on the image.
  9. Click the Add Control Point tool and with this tool click once on the image to be georeferenced (the “Source”) control point and again on the same location in reference layer (the “Map”). A set of Source and Map coordinates will appear in the Links Table. If the reference point on the Map is not visible after the first click (e.g. you are zoomed in too far) simply click the demagnifier, back arrow or any tool on the Tools toolbar to adjust the view, then click the Add Control Point tool again and click the now visible reference point.
  10. Continue this process to establish a minimum of three control points. With more than three, an RMS error (visible in the Links table) can be used to gauge the goodness of fit. It is sometimes useful to first rapidly establish three points, then carefully search for additional points that might be more precisely located on both the image and the reference data. When such points are successfully located, delete earlier points (highlight the line in the Links table and click the X button on the upper right) to lower the RMS error and more precisely locate the image.
  11. When the image is located to your satisfaction you can save the Link table by clicking the Save button at the bottom of the link table window. (Should you mess up and later have to relocate control points, you could simply Load these points into a new Link table.)
  12. To create a new file that is spatially referenced (so you’ll never have to do this again for this image…), from the georeferencing toolbar drop-down menu select “Rectify…”, fill in the dialog box (see the lecture notes for details) and Apply.
  13. Although the image is now saved with geographically meaningful coordinates, the coordinate system may still be undefined. To check whether it is and to define it if it isn’t, follow the two steps at the top of this tip. The coordinates will be in the coordinate system of the Data Frame (see step 2).

An excellent, well illustrated georeferencing tutorial is available courtesy of the National Geologic Map Database, US Geological Survey.

   
       

Last updated September 13, 2011
Comments and questions to helper@mail.utexas.edu
Geological Science,  U. Texas at Austin