Using the Raster to Polygon Tool on a Scanned Map

kzuj21 — Tue, 14 Dec 2021 08:29:40 GMT

Hi everyone,

Sorry up front if this is posted in the wrong section. I'm new to this forum and not an arcgis expert in general. I could not find an ideal place to post this.

I am attempting to take a scanned TIFF geologic map, that I georeferenced, and use the Raster to Polygon tool on it. Ideally to break the map into useable polygons. I am using Arcgis 10.8.1. (not Pro).

While this seems like a simple task, I have found suprisingly little information online about this. Maybe because it isn't easily possible, and digitizing everything by hand is more precise.

Below is what I tried, but I have a feeling there may be a better way to go about this.

First I clipped out a smaller section of my map because the map was simply too large to use the Raster to Polygon tool.

I determined that using the Reclassify tool first may be a good way to reorganize my data. Of course, this breaks the map into components based on color. However, I am a little confused by what value arcgis using exactly to divide the image, since there are technically three colors (red, green and blue) but a single number is taken into account using the Reclassify tool.

Anyways, using this tool produced a map with multiple values within what is supposed to be a single color (on the original TIFF), which is not what I want. Ideally one color for each color on the legend of the original geologic map. Ideally as a single polygon for each region. I tried to edit a few minor settings in arcgis to fix the image quality to cut down on this, but nothing quite did the trick.

I then determined that maybe breaking down the color palette into fewer components may help solve the problem. So I exported the map as a PNG (while selecting "use renderer" and "force RGB" boxes) and imported it into Photoshop where I used "index values" to reduce the quantity of colors in the image. I tried experimenting with Dust and Scratch as well, but was less happy with the result.

I then imported this back into arcmap, reclassified it (giving each color a unique value), then used the "raster to polygon" tool. This improved the results, however there were single polygons scattered throughout the map (but fewer than I saw before I simplified the color palette). I then began merging polygons together to make it resemble the original map.

So maybe to acheive what I'm looking for Digitizing is the best option (hence, why there is no information about this online). I am very interested in hearing the community's thoughts!

Re: Using the Raster to Polygon Tool on a Scanned Map

DanPatterson — Tue, 14 Dec 2021 11:25:21 GMT

I suspect you need Overview of image classification—ArcGIS Pro | Documentation

a scanned tiff is no more than a picture, it has no inherent sense of what the colors mean, that is for the user to provide

topic Re: Using the Raster to Polygon Tool on a Scanned Map in ArcGIS Spatial Analyst Questions

Using the Raster to Polygon Tool on a Scanned Map

Re: Using the Raster to Polygon Tool on a Scanned Map