topic Re: Extract colored polygons from raster image in ArcGIS Pro Questions

Extract colored polygons from raster image

Bud — Wed, 09 Nov 2022 17:19:43 GMT

I have a raster map that has red areas in it. I want to extract the red areas as a polygon FC for spatial analysis purposes (find where the red polygons overlap a different polygon FC).

Unfortunately, the source vector data isn't available from the government organization that produced the map (1980). So I'm hoping to extract the red areas using GIS tools instead.

I've georeferenced the image in ArcGIS Pro. Next, I think I need to:

Remove pixels that aren't red.
Convert the red pixels to polygons.

What's a good way to do that?

ArcGIS Pro 2.6.8; Spatial Analyst Extension

Re: Extract colored polygons from raster image

DanPatterson — Wed, 09 Nov 2022 20:04:51 GMT

Is there only one red? or a range of red colors?

If you know its class value, you can extract by its value

Extract by Attributes (Spatial Analyst)—ArcGIS Pro | Documentation

then use

Raster to Polygon (Conversion)—ArcGIS Pro | Documentation

ensuring that you use the same raster extent, cell size and alignment as the input raster.

The generated polygons might not be to your liking, so you could convert your other polygon file to raster and do the analysis within the spatial analyst (eg Combine etc)

Re: Extract colored polygons from raster image

Bud — Fri, 11 Nov 2022 03:21:27 GMT

Here's what I have so far:

In Adobe Photoshop 2023:

Select by Color Range to select the reds (there are multiple red colors).
Fill in the reds with a solid color.
Inverse the selection to select everything else -- and delete those pixels.
Manually erase the numbers/labels. (I actually did it Microsoft Paint, not Photoshop. But I don't think it matters.)
Boost the Contrast way up to fill in the light spots.
Boost the Exposure way up to get rid of the noise/spots.

I did many of those steps multiple times to get the desired effect.

Now, only the red areas are shown, as a single color: R:255, G:0, B:0.

Fix the georeferencing. It turns out it wasn't as well aligned as I thought.

Having trouble fixing it -- I can't seem to get the entire map to be properly aligned.