Merge polygon without attribute (in field) with nearest polygon with attribute

Wrangling Polygons: A Friendly Guide to Combining Spatial Data Like a Pro

Ever felt like your GIS data is a bit of a mess? Maybe you’ve got tiny, annoying sliver polygons cluttering up your map, or perhaps some areas are missing key information. Don’t worry, we’ve all been there! One of the handiest tricks in the GIS toolkit is merging polygons – especially when you need to combine a polygon that’s missing a certain piece of information with its closest neighbor that does have it. Think of it as tidying up your spatial data and giving it a bit of a brain boost.

Why Bother Merging Polygons Anyway?

So, why should you care about merging polygons? Well, it’s surprisingly useful for a bunch of things:

• Cleaning up messy data: Get rid of those pesky little sliver polygons that can appear after digitizing. They’re like digital dust bunnies!
• Simplifying complex maps: Sometimes, less is more. Merging polygons can help you create a simpler, easier-to-understand map by combining smaller areas into larger ones.
• Filling in the blanks: Got a polygon with missing information? Merge it with a neighbor that does have the data, and bingo, problem solved!
• Creating bigger, better areas: Imagine you’re mapping forest types. If you have a bunch of small, adjacent polygons with the same forest type, you can merge them into one big, contiguous forest.

How to Actually Merge These Things

Okay, let’s get down to the nitty-gritty. There are several ways to merge polygons, and the best approach depends on what you’re trying to achieve and the software you’re using. Here are a few of my favorite methods:

1. Dissolve: The Great Unifier

The Dissolve tool is a classic for a reason. It’s like the ultimate peacemaker, combining adjacent polygons based on shared characteristics or just because they’re next to each other.

• Dissolve by Attribute: Got polygons that share a common attribute, like “residential” land use? Dissolve them and create one big residential area!
• Dissolve by Proximity: Just want to get rid of internal borders? Dissolve will merge any polygons that share a boundary. Simple as that.

2. Spatial Join: Matchmaker, Matchmaker

Think of Spatial Join as a dating app for spatial data. It lets you combine information from two different layers based on their location. This is perfect for when you want to transfer attributes from one layer to another based on where they are.

• Find the Nearest Neighbor: Spatial Join can pinpoint the closest polygon with the attribute you need and copy that attribute over to your target polygon. It’s like saying, “Hey, you’re close enough, you get the data!”
• One-to-One or One-to-Many? Choose “one-to-one” if you only want to grab the attribute from the absolute closest polygon. “One-to-many” is for more complex scenarios where you might want to consider multiple nearby polygons, but that’s a story for another day.

3. Eliminate: The Polygon Vacuum

The Eliminate tool is specifically designed to suck up those small, unwanted polygons and merge them with their bigger, more important neighbors.

• Be Picky! The “exclusion expression” option lets you tell Eliminate which polygons not to touch. This is super handy when you only want to merge polygons that are missing a specific attribute.
• How to Choose a Neighbor: Eliminate usually merges based on either the largest area or the longest shared border. It’s like saying, “Okay, you’re the biggest and closest, you get to absorb this little guy.”

4. Buffer and Dissolve: Bridging the Gap

This technique is a bit more involved, but it’s great for merging polygons that are close to each other but don’t actually touch.

5. Aggregate Polygons: Gathering the Troops

The Aggregate Polygons tool is similar to Buffer and Dissolve, but it’s specifically designed for combining polygons that are clustered together.

• Set the Aggregation Distance: This tells the tool how close polygons need to be to be considered part of the same “group.”
• Mix and Match: Combine Aggregate Polygons with Spatial Join and Dissolve for even more control over the merging process.

Let’s Walk Through an Example

Okay, let’s say you have a bunch of polygons, and some of them are missing a “land use” attribute. Here’s how you could merge those polygons with their nearest neighbor that does have the land use information:

Software Quirks

Keep in mind that the exact names of these tools might be slightly different depending on whether you’re using ArcGIS Pro, QGIS, or some other GIS software. But the basic concepts are the same across the board.

The Takeaway

Merging polygons is a super-useful skill for anyone working with GIS data. It’s like having a digital broom and dustpan, letting you clean up messy data, fill in missing information, and create more useful and informative maps. So go forth and merge! Your data will thank you for it.