Calculating areas of polygon located inside target polygon in ArcMap?

Figuring Out Polygon Areas Inside a Target Zone in ArcMap: A Down-to-Earth Guide

Ever needed to know the area of, say, parks within a specific neighborhood using ArcMap? It’s a pretty common task for GIS folks, whether you’re into environmental studies, city planning, or managing resources. Basically, you’re trying to figure out how much of one thing falls inside another. Sounds simple, right? Well, it can get a little tricky if you don’t know the ropes. Just a heads-up, though: ArcMap is getting a bit long in the tooth and will be retired on March 1, 2026. So, while we’re diving in here, it might be worth thinking about making the jump to ArcGIS Pro down the line.

The Head-Scratcher

The real challenge isn’t just slapping one layer on top of another. It’s about getting accurate measurements of those pieces of polygons that are neatly tucked inside your target area. If your initial polygons stretch beyond that target, a simple overlay just won’t cut it.

The Nitty-Gritty: A Step-by-Step Approach

The best way to tackle this? Think of it like this: first, you carve out the bits you need, then you measure them. The “Intersect” tool is your carving knife, and calculating geometry is your measuring tape.

Beyond Intersect: Other Tools in Your Arsenal

While Intersect is a go-to, there are other ways to skin this cat, depending on what you need:

• Clip Tool: Think of this as a more straightforward cookie cutter. It chops off anything outside your target area. But remember, if you’re using shapefiles, you’ll need to manually recalculate the area afterward.
• Spatial Join: This is like saying, “Hey, which polygons are inside this target, and what are their areas?” It lets you transfer attributes based on location.
• Summarize Within: Need a quick summary of what’s inside? This tool gives you stats, including the total area of features within your target zone.
• Tabulate Intersection: This one’s a bit more advanced. It not only intersects the features but also cross-tabulates the area, length, or count of the intersecting bits. It’s great for figuring out how much of each feature type falls within your target area.

Keep These Gotchas in Mind

• Coordinate Systems are Key: I can’t stress this enough: always, always use a projected coordinate system when calculating areas. Geographic systems are a no-go for accurate measurements.
• Overlapping Polygons? Careful! If your starting layer has polygons that overlap, you’ll end up double-counting areas. Use the Dissolve tool beforehand to merge those overlaps into single features.
• Data Types Matter: When you create that new area field, “Double” is your friend. It handles those decimal places like a champ.
• Shapefiles vs. Geodatabases: Shapefiles are a bit old-school. If you’re using them, remember that the area doesn’t automatically update after you edit or clip. You’ve got to recalculate it. Geodatabases, on the other hand, are smarter and update that SHAPE_Area field automatically.
• Multipart Features: When dissolving those overlapping polygons, make sure you don’t create multipart features unless you really mean to. Otherwise, you might end up merging polygons that just touch but don’t actually overlap.

Real-World Example

Let’s say you’re a city planner trying to figure out how much green space is in a particular district. You’ve got a layer showing the district boundary and another layer showing all the parks. Intersect those layers, calculate the geometry, and you’ve got the area of each park within the district. Add those up, and you know exactly how much green space that district boasts.

The Takeaway

Calculating polygon areas inside a target area in ArcMap boils down to using the Intersect tool and the Calculate Geometry function. Pay attention to coordinate systems, data types, and potential overlaps, and you’ll be golden. And hey, don’t forget to peek at ArcGIS Pro – it’s the future!