ArcMap to automatically create polygons from shapes that are present in an image raster

Turning Raster Images into Polygons in ArcMap: A Human’s Guide

So, you’ve got a raster image – maybe it’s a scanned map, some aerial photography, or even classified data – and you need to turn those shapes into polygons. Sounds familiar, right? As GIS folks, we’ve all been there. Converting raster images into vector polygons is a bread-and-butter task. And let me tell you, being able to automate this process in ArcMap? It’s a game-changer. It can seriously streamline your workflow. Let’s dive into how it’s done, with a few tips I’ve picked up along the way. Quick note though: ArcMap is getting a bit long in the tooth and will be retired on March 1, 2026. Esri is nudging everyone toward ArcGIS Pro, so keep that in mind!

Raster vs. Vector: A Quick Refresher

Before we get our hands dirty, let’s quickly recap the difference between raster and vector data. Think of raster data like a digital photo: it’s a grid of pixels, each with a value. Vector data, on the other hand, uses points, lines, and polygons to represent things. Vector’s great because you can zoom in forever without losing quality, and it’s perfect for representing distinct objects with clear boundaries.

Why convert? Well, vectorizing lets you take advantage of all the good stuff vector data offers: analysis, editing, and making beautiful maps. Trust me, it’s worth the effort.

ArcMap’s “Raster to Polygon” Tool: Your New Best Friend

ArcMap has a tool specifically for this: the “Raster to Polygon” tool. You’ll find it hanging out in the Conversion toolbox. This little gem automatically creates polygons from those connected raster cells that share the same value. Pretty neat, huh?

Here’s how to use it, step-by-step:

Pro Tips for a Smooth Conversion

• Clean Up Your Image First: Trust me on this one. Cleaning up your raster before converting it makes a huge difference. Get rid of any junk, tweak the contrast, and reduce noise. It’ll save you headaches later.
• Resolution Matters: Think about the resolution of your raster. A higher resolution means more detail in your polygons, but it also takes longer to process. I usually aim for around 300 DPI for detailed conversions.
• Georeference It: If your raster isn’t already georeferenced, you gotta do it. This aligns the raster to a real-world coordinate system.
• Simplify Wisely: If you’re simplifying polygons, play around with the tolerance. It’s a balancing act between accuracy and speed.
• NoData is Your Friend: NoData cells won’t be converted, so use that to your advantage. Make sure to define “NO VALUE” in the mosaic to avoid the background of raster from being included in the polygon.
• ArcScan for the Win (Sometimes): For really tricky vectorization, especially with scanned documents, check out the ArcScan extension. It’s got some cool interactive and automated tools, like centerline and outline vectorization. Just a heads-up: ArcScan isn’t in ArcGIS Pro.

Troubleshooting Common Issues

• Polygons Are Offset: This can happen. Make sure all your layers are in the same coordinate system. Also, double-check the environment settings in the Raster to Polygon tool. There’s a bug in the software, so make sure that the “Create MultiPart Features” option is deselected.
• It’s Taking Forever: Converting huge rasters can be a drag. Simplify the raster, use a coarser resolution, and turn on polygon simplification to speed things up.
• Stair-Stepping: See those jagged edges? That’s the “NO_SIMPLIFY” option in action. You can smooth them out later with a smoothing algorithm.

Beyond the Conversion: What’s Next?

Once you’ve got your polygons, the real fun begins! You can do all sorts of spatial analyses: calculate areas, measure distances, overlay them with other data. And, of course, you can always edit the polygons to fix any mistakes or make them just right.

Final Thoughts

Converting rasters to polygons in ArcMap is a skill every GIS pro should have. It unlocks so much potential for working with spatial data. So, give it a try, experiment with the settings, and don’t be afraid to get your hands dirty. And remember, ArcGIS Pro is the future, so start thinking about making the switch!