Georeferencing is the process of associating geographic data in one GIS layer with known locations in another GIS layer. Accordingly, when georeferencing, two map layers are needed:
- the layer to be georeferenced (usually an aerial photograph or scanned map); and
- the reference layer (typically a basemap).
Map the S: drive if needed.
Open ArcGIS Desktop and open your .MXD file or create a Blank Map.
Choose Customize/Toolbars/Georeferencing to open the Georeferencing Toolbar. You may dock the toolbar if you wish.
In the procedure, below, the default projection for a basemap (Web Mercator) is used for georeferencing. If you wish to use a different projection, you will need to specify that projection before adding the layer to be projected.
Click to view the screenshot.
Click the Add Data button and choose Add Basemap…
Add an appropriate basemap to your map.
- Topographic is useful if you are georeferencing a scanned map.
- Imagery is useful if you are georeferencing an aerial image.
Next, do one of the following:
- Click the Add Data button and choose Add Data… Navigate to the image you wish to georeference. Click Add.
- Use the Catalog window to drag an image to your map.
Zoom into the area of the basemap the scanned map or aerial image represents. It is useful to bookmark that zoom extent.
Click the Georeferencing dropdown menu and choose Fit to Display. The scanned or aerial image will be fit to the current map extent.
By zooming, adjusting the map extent, and choosing Fit to Display, gradually fit the image to your map extent.
Save your map.
Click the Add Control Points tool on the Georeferencing Toolbar.
Add control points as follows:
- Click a known location on the image.
- Find the same location in the basemap and click it.
Repeat this process until you have added at least four control points.
Save your map frequently.
Tip 1: Look for road intersections, land features, building corners, and other features that are common to both the image and the basemap.
Tip 2: If you wish to delete an unwanted control point:
- Click View Link Table on the Georeferencing Toolbar.
- Choose the control point to be deleted.
- Touch the Delete key.
Tip 3: When working with an image, you may wish to adjust its transparency and/or turn the image layer off and on as you add control points.
- Right-click the layer you wish to adjust and choose Properties…
- Click the Advanced tab and move the Layer Transparency slider.
- Click Apply to view the effect of the transparency setting on the layer.
- Click OK when done with adjusting the layer transparency.
Tip 4: Spread your control points across the image. Don’t focus on one area.
Tip 5: Search for historic maps to use as basemaps in ArcGIS Online.
- Choose Add Data From ArcGIS Online.
- Search for “Historic” maps (or use another relevant search term such as “Rumsey”).
- Click Add to add a layer to your map.
- If necessary, drag the new layer below the image layer to be georeferenced.
Saving the Georeferenced Image
Click the Georeferencing dropdown menu and choose Update Georeferencing. That will apply the transformation to the image.
To save a copy of the georeferenced file with the projection information:
- From the Georeferencing dropdown menu, choose Rectify…
- In the resulting dialog box, (1.) accept the default cell size, NoData value, and resample type (Nearest Neighbor); (2.) ensure that the path to save the new file is correct; and (3.) specify a file name for the file. Then, click Save.
- After the saving process is complete, in the data folder you specified, you will see the image file you georeferenced as well as additional files that provide georeferencing information to ArcGIS. If you wish to move the georeferenced image to a new location on your computer, all of the associated files must be moved as well.
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