0

I am georeferencing a historical map and then trying to harmonise it with polygons from another file. I am trying to think about the best way to do this with respect to setting control points.

To give you an idea, I have a map with polygons circa 1900, and a shapefile of current polygons in 2020. Due to various reasons, the size of the polygons has changed over this period. I want to create a new shapefile using shapes from the 1900 map, but locations from the 2020 shapefile (which is from the U.S. Census Bureau and is therefore the 'truth'). Below you can see an example of a quickly georeferenced map (using lon / lat), and the 2020 polygons. You can see that 641 is unchanged from 1900 to 2020, but 582 is smaller in 2020 than it was in 1900.

What would be the best practice here? I am thinking of setting control points on the 2020 polygons themselves (the ones that have not changed shape) with a view that if these match, the locations of polygons such as 582 would be 'correct' when I get around to digitising them. I am not experienced enough to know if this is a bad practice.

enter image description here

Improve this question
2
  • 2
    It looks like you have public land survey townships on your old map. Why not use those corners along with a modern PLS data layer for controlling the georeferencing?
    – GBG
    Commented Jun 25 at 18:15
  • I hadn't thought of this. It is a good idea, and at this early stage it seems to be helping a lot. Thank you for the suggestion!
    – Cola
    Commented Jun 26 at 11:04

1 Answer 1

Reset to default
0

Your map has latitude and longitude lines so this is an easy task.

Open your old map in an image viewing software like Gimp (Freeware) or Photoshop.

Mouse over the image and place your cursor on the intersections of the lines of latitude and longitude. Enter the image x and y coordinate values in the georeferencer GCP table as shown in Gimp. Enter the latitude and longitude as shown on your map into the georeference GCP table. Repeat until you have enough points for your chosen georeferencing transformation choice.

Improve this answer
1
  • The issue I had is that there is only a handful of intersections per map (since there is one per state). So even after georeferencing with lat-lon lines, there are discrepancies between the polygons in the 2020 shapefile and the same ones on the historical map. However, I did not think of using PLSS intersections as you suggested, and I think this will help with the problem. Thank you!
    – Cola
    Commented Jun 26 at 11:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.