Substracting rasters using ArcGIS Raster Calculator?

Raster Subtraction with ArcGIS Raster Calculator: A Human’s Guide

Ever needed to spot changes in a landscape over time, or maybe figure out how much a hillside eroded after a storm? That’s where raster subtraction comes in, and the ArcGIS Raster Calculator is your trusty sidekick. It’s like doing math, but with maps! This tool lets you perform cell-by-cell calculations on raster datasets, and believe me, subtracting rasters is one of the most useful tricks in the book.

Raster Subtraction: What’s the Big Deal?

Think of raster subtraction as comparing two images of the same place, but taken at different times. You’re essentially subtracting the value of each cell in one raster from the corresponding cell in another. The result? A brand-new raster that highlights the differences. This simple operation unlocks a ton of possibilities:

• Spotting Change: Imagine you have satellite images of a forest from 2010 and 2020. Subtract one from the other, and boom! You can instantly see where deforestation has occurred or where new growth has sprung up.
• Mapping Erosion: Got elevation data (DEMs) before and after a landslide? Subtract them, and you’ll get a map showing exactly where material was lost (erosion) and where it piled up (deposition).
• Isolating Features: Sometimes, you want to focus on specific terrain features. For instance, subtracting a raster of water bodies from a DEM can help you isolate things like ridges and valleys.
• Cleaning Up Images: Atmospheric haze can really mess with satellite imagery. Subtracting a “haze raster” can help clear things up, giving you a much cleaner picture.

Cracking Open the Raster Calculator

Finding the Raster Calculator is pretty straightforward:

Let’s Do Some Subtraction!

Alright, here’s the step-by-step on how to actually subtract two rasters:

The resulting raster will show you the cell-by-cell difference between your two input rasters. Easy peasy!

Map Algebra: It’s Like a Secret Language

The Raster Calculator speaks Map Algebra, which is basically a way of writing mathematical expressions for spatial analysis. Here’s the cheat sheet:

• Just use the raster layer names directly in your expressions.
• The usual suspects (+, -, *, /) do cell-by-cell operations.
• Don’t include an output name or equals sign in your expression.
• Remember, Python is case-sensitive, so get those tool names right!

So, if you want to subtract “Raster_A” from “Raster_B”, your expression would simply be:

“Raster_B” – “Raster_A”

A Few Things to Keep in Mind

• Data Types Matter: If you’re using integer rasters, the output will also be an integer. If you have floating-point rasters, the output will be floating-point. Choose wisely to avoid losing precision!
• Size and Alignment: Make sure your rasters have the same cell size and line up properly. If not, you might need to resample one of them using the “Resample” tool.
• NoData Caveats: NoData values can be tricky. If a cell has NoData in either input, the output will usually also be NoData. You can use the “Con” tool to replace NoData with a value like 0 if that makes more sense for your analysis.
• Extent Control: By default, the tool only processes the area where the rasters overlap. If you want to keep the full extent of the larger raster, tweak the environment settings to use the “Union” of extents.
• Multidimensional Rasters: Got multidimensional data? Make sure your variables play nicely together – they need to have the same dimensions or at least share some common ones.

Beyond the Raster Calculator

While the Raster Calculator is a workhorse, there are other ways to skin this cat:

• The Minus Tool: The “Minus” tool in the Spatial Analyst toolbox does the same thing as subtraction in the Raster Calculator. It’s handy for building models in ModelBuilder.
• Image Analysis Window: The Image Analysis window has a “Difference” function for quickly visualizing differences between rasters.

Final Thoughts

Subtracting rasters with the ArcGIS Raster Calculator is a fundamental skill for anyone working with spatial data. Master this technique, and you’ll be able to unlock insights from your raster datasets that you never thought possible. So, go ahead, give it a try! You might be surprised at what you discover.