Generate Contour Lines from Georeferenced Map Image

Question

Hello! I'm trying to figure out a way around having to hand trace contour lines on a Georeferenced map image with contour lines, which will be used to create a raster elevation surface.

Steps so far:

-Select the image in contents pane-->right click-->data-->export raster

-Raster to Polygon or Raster to Polyline

-The tools will generate polygons from the contour lines, but each line is over 1000 different small polygons.

I have also tried the Generate Trend Raster tool, but had the same outcome.

Thanks for the help!

MattieFriday1 · Answer

M- I am hand digitizing the contour lines after georeferencing the image, but was attempting to programmatically detect them in hopes there is a fast way that clicking on every contour line that I need.

Re: "small polygons" if you zoom in to one of the contour lines that was extracted from the image you see that it's not actually a solid line, but 1000s of small connected polygons. Which makes me think your point about the "visual raster" is correct.

I mostly do this with smaller state-sized areas so a extraction solution may be more tedious than tracing the contour lines. Wishful thinking!

MattieFriday1 · Answer

Thank you, David! After trying a few different ways I came to the same conclusion about it being not worth the effort, particularly if it is a small area.

I'll try your recommendations and see if it works!

DavidPike · Answer

It's tricky and maybe not worth the effort depending on your goals.  I'd look first to see if you can identify certain colours which are only used for the contours.  This is probably hopeful but worth a shot.

Then https://pro.arcgis.com/en/pro-app/latest/tool-reference/spatial-analyst/reclassify.htm, setting the other values to NoData.

https://pro.arcgis.com/en/pro-app/latest/tool-reference/conversion/raster-to-polyline.htm Raster to Polyline (use simplified) to get the contours.

You'd then have to label/attribute the contours manually and probably do alot of manual cleanup editing as the colours are probably used elsewhere in the map, and for the original raster contour labels.

I'd recommend looking for a DEM and generating some fresh contours off those and see how they line up instead.