on December 26, 2022

Geometry checker not removing overlap between features

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Geometry Checker Not Removing Overlap Between Features: What’s Going On?

Let’s face it: in the GIS world, data accuracy is king. You can’t make good decisions with bad data, right? And one of the sneakiest culprits messing with our data? Overlapping features, especially when we’re talking about polygons. These overlaps can throw off everything from simple area calculations to complex spatial analyses. Geometry checkers are supposed to be our trusty sidekicks in this fight, designed to spot and fix these issues automatically. But what happens when they just…don’t? When they leave those overlaps stubbornly in place? That’s what we’re diving into today.

Why Bother with Clean Geometry Anyway?

Before we get into the nitty-gritty, let’s quickly recap why clean geometry matters in the first place. Think of it this way: overlapping polygons are like double-dipping at a party – a big no-no. They can:

  • Muddle Area Calculations: Imagine trying to figure out the total area of forest cover, but some areas are counted twice because of overlaps. Not good!
  • Wreck Spatial Analysis: Those cool tools we use for analysis, like union or intersection? Overlaps can make them spit out totally wonky results.
  • Confuse the Eye: A map with overlapping features looks messy and can lead to serious misinterpretations. Nobody wants that.
  • Break the Rules: Overlaps violate those fundamental topological rules that keep our spatial data in order.

So, Why Do Geometry Checkers Sometimes Drop the Ball?

Okay, so you ran your geometry checker, and…nothing. Those overlaps are still glaring back at you. What gives? Well, there are a few common reasons why these tools might fail to do their job:

  • Tolerance Troubles: Geometry checkers have this thing called “tolerance,” which is basically how close two points need to be before they’re considered the same. If your overlap is smaller than the tolerance setting, the checker might just miss it. It’s like trying to catch tiny crumbs with a big net.
  • Complexity Overload: Some overlaps are just plain nasty – complex shapes, multiple features involved, the whole nine yards. Sometimes, the automated algorithms just can’t handle the complexity.
  • Data Type Drama: Not all geometry checkers are created equal. Some have limits on the types of data they can handle or the kinds of errors they can fix. For example, it might struggle with polygons that cross themselves.
  • Underlying Issues: Sometimes, the problem isn’t just the overlap itself, but something deeper in the geometry. If the polygon is fundamentally invalid (like having lines that cross each other), the checker might not be able to fix the overlap until that’s sorted out first.
  • Software Quirks: Let’s be honest, every GIS software package has its own little quirks. What works like a charm in ArcGIS might not work at all in QGIS, and vice versa.
  • Wrong Tool for the Task: This is a big one! Sometimes, you’re simply using the wrong tool for the job. A geometry checker might not be the best solution when a spatial overlay tool like “Union” or “Erase” is what you really need. It’s like using a screwdriver to hammer a nail.

    • Time to Get Those Overlaps Sorted!

    Alright, so your geometry checker isn’t cutting it. Don’t despair! Here’s a toolkit of strategies you can use to tackle those overlaps head-on:

  • Tweak That Tolerance: Play around with the tolerance setting in your geometry checker. A smaller tolerance can catch those tiny overlaps, but be careful not to go too small, or you might end up creating new problems.
  • Get Hands-On: For small, localized overlaps, sometimes the best approach is to just manually edit them. Make sure snapping is turned on to keep everything aligned.
  • Lay Down the Law (Topology Rules): If you’re working in a geodatabase, set up topology rules to define how your features should relate to each other. Then, validate the topology and use the editing tools to fix any violations. Think of it as setting the ground rules for your data.
  • Overlay Power: Spatial overlay tools like “Union,” “Intersect,” and “Erase” are your friends here.
    • Union: Combines layers, resolving overlaps based on layer order or attributes.
    • Intersect: Finds the areas where features overlap, creating new features.
    • Erase: Removes the overlapping parts of one layer from another.
  • “Fix Geometry” to the Rescue: Most GIS software has a “Fix Geometry” tool that tries to repair invalid geometries. Running this before anything else can make a big difference.
  • Dissolve the Problem: The Dissolve tool merges polygons that share a common attribute, getting rid of overlaps along the way.
  • Buffer, Then Dissolve: Try buffering your polygons and then dissolving the result. This can turn empty spaces into overlaps that are easier to deal with.
  • PostGIS to the Rescue: If you’re using PostGIS, dive into those functions like ST_MakeValid and ST_Overlaps. They’re powerful tools for finding and fixing geometry issues.
  • Sliver Patrol: Hunt down those sliver polygons – those tiny, annoying overlaps or gaps that often come from digitizing errors.
  • Prep Your Data: Before you even start checking for overlaps, make sure your data is clean. Remove duplicates, handle missing values, and convert everything to a compatible format.
  • Algorithm-Based Verification: Use automated systems to process thousands of map features per minute, checking for all sorts of errors.
  • Look to the Past: Compare your current data to older, trusted datasets to make sure everything lines up and nothing’s out of date.

    • A Few Software-Specific Tips

    • ArcGIS: Get cozy with geodatabase topology rules, the “Check Geometry” and “Repair Geometry” tools, and the Data Reviewer extension.
    • QGIS: Check out the Topology Checker plugin, the “Fix geometries” algorithm, and tools like “Dissolve” and “Buffer.”
    • PostGIS: Write some SQL queries using spatial functions like ST_MakeValid, ST_Overlaps, and ST_Union.

    Stop Overlaps Before They Start

    Of course, the best way to deal with overlaps is to prevent them in the first place. Here are a few tips:

    • Snap to It: Always use snapping tools when you’re digitizing or editing.
    • Edit with Topology in Mind: When you’re editing shared boundaries, do it within a topology to keep things consistent.
    • Validate Regularly: Make data validation a regular part of your workflow.
    • Quality Control is Key: Set up a quality control process to review and correct data before it goes live.

    The Bottom Line

    Geometry checkers are great, but they’re not a magic bullet. When they don’t quite do the trick, it’s time to roll up your sleeves and use a combination of techniques to get those overlaps under control. And remember, a little prevention goes a long way! By following best practices for digitizing and editing, you can save yourself a lot of headaches down the road. After all, clean data means accurate analysis, reliable results, and better decisions. And that’s what we’re all after, right?

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