Counting occurrences across many rasters using ArcGIS Desktop?

Counting Stuff Across a Bunch of Raster Maps in ArcGIS Desktop: A Real-World Guide

Ever found yourself staring at a stack of raster maps, wishing you could just… count something? Like, how many times a specific thing pops up in the same spot across all those maps? It’s a surprisingly common problem in geospatial analysis, and ArcGIS Desktop has some tricks up its sleeve to help you out.

Let’s say you’re tracking drought severity over a decade. You’ve got a map for each year, showing how bad the drought was in different areas. What if you wanted to know how many times a particular spot experienced a “severe” drought over those ten years? That’s where counting occurrences comes in. You wouldn’t want to manually inspect each map, would you?

ArcGIS Desktop, while getting a bit long in the tooth (it’s being retired in 2026, so Esri is nudging everyone towards ArcGIS Pro), still packs a punch for this kind of work. The Spatial Analyst extension is your best friend here.

So, how do we actually do it? Here’s the lowdown:

First, get your ducks in a row. That means prepping your data.

• Rasterize Vectors (if needed): If your data’s in shapefiles, convert them to rasters. Think of it like turning a drawing into a pixelated image. Make sure all your rasters line up perfectly – same size, same grid, same everything.
• Reclassify (maybe): Sometimes, you need to simplify your data. If your drought index is all over the place, reclassify it into a simpler scale, like 1 to 5.
• NoData? No Problem: “NoData” values can mess things up. Turn them into something useful, like zero, using the Reclassify tool. It’s like filling in the blanks so the calculations don’t get confused.

Now for the fun part: counting!

• The “Equal To Frequency” Tool is Your Pal: Seriously, this tool is made for this. It checks each cell across your stack of rasters and counts how many times it matches a specific value. Think of it as a super-efficient map checker.
• Rinse and Repeat: You’ll need to run the “Equal To Frequency” tool for each thing you want to count. Want to know how many times drought level 1, 2, and 3 occurred? Run it three times, each time telling it to look for a different drought level. To specify the value you are looking for, you can create a constant raster of that value using the Create Constant Raster tool.

Here are a couple of other ways of doing it:

• Cell Statistics: The Cell Statistics tool can also be used to calculate statistics on a cell-by-cell basis across multiple rasters.
• Zonal Statistics: Zonal statistics summarize the values of raster cells falling within zones defined by another dataset.

Let’s walk through an example. Imagine those 10 drought maps, with values from 1 to 5. To count how many times drought level 3 shows up:

A few things to keep in mind:

• Spatial Analyst is a Must: You can’t do this without the Spatial Analyst extension. It’s like trying to bake a cake without an oven.
• Data Types Matter: Stick to integer rasters for counting. They’re cleaner and faster.
• NoData is the Enemy: Seriously, handle those NoData values. They’ll skew your results if you don’t.
• Big Data = Big Processing: Large rasters can take a while to process. Optimize your cell sizes and extents to speed things up. Parallel processing can also help.

So, there you have it. Counting occurrences across multiple rasters in ArcGIS Desktop isn’t rocket science, but it’s a powerful technique. Whether you’re tracking droughts, monitoring land use, or studying climate change, this method can help you unlock valuable insights from your geospatial data. And even though ArcGIS Desktop is on its way out, these skills will translate nicely to ArcGIS Pro, so you’re not wasting your time learning them.