Georeferencing

2015-Sep-03

A case study in georeferencing with ArcGIS 10.3.1

ESRI's documentation is here.

I did georeferencing with old black and white aerial photos a long time ago. Around 2004, so ArcGIS 8.2.

I am trying it with a paper map today, just for kicks. It's a Northern California bicycle map, and it looks almost hand drawn, so I am curious how it will align. I am scanning the Eureka/Arcata area.

Sample image

Scanner settings: full color, 600 DPI. TIFF format.

Here is a scaled down version of the scan for your edification.

The process

1. Scan the map.
2. Grab some data to which to tie the scan. Just need roads; also grabbed coastal zones. http://www.humboldtgov.org/276/GIS-Data-Download
3. Start ArcMap and create a map with roads and coastal zones.
4. Add the unreferenced photo as a layer. Ignore the complaints about it being unreferenced. :-)
5. Open the georeferencing toolbar. The photo should already show in there.
6. Zoom to the approximate area of the map. (It helps to have it open in a TIFF viewer.)
7. In the toolbar, select "Fit to display".
8. Pick some nice solid control points, like major highway intersections.
9. Using the tools create links from the scan to the vector data.
10. I chose "Update Georeferencing" to save the references with the TIFF instead of Rectify which would generate a new image. This also means it will probably only work correctly in ArcMap, and maybe QGIS.

Note that Humboldt county is still on NAD27. NAD_1927_StatePlane_California_I_FIPS_0401

After doing the georeferencing I think the cartographer pulled the data from the same projection; it only required 3 points to pull it into alignment.