Unless you’re a data editing jedi, your GIS data may unknowingly have errors like gaps and overlaps… What to do? Use topology rules in ArcGIS:
If you answer yes to any of the following questions, you should consider using GIS topology rules to check to validate and fix errors in your GIS data.
- Do unwanted gaps exist in your data?
- Do overlaps exist?
- Should lines touch the boundary of polygons?
What is topology in GIS? Topology is a set of rules how points, lines and polygons share their geometry. Topology checks and validates the spatial relationship of neighboring and overlapping features.
Types of Errors: Accuracy vs Precision
Analysts often assess data validity using accuracy and precision – which aren’t the same. What’s the difference between accuracy of precision?
- Precision identifies the closeness of two or more measurements to each other.
- Accuracy refers to the closeness of a measured value to a known value.
Unknowingly, errors can exist in GIS data. You may not even know. How scary is that?
That’s why we can fix errors automatically with topology rules. Sources of data error can be introduced from positional accuracy, digitizing error, data generalization and age of data:
Overlaps are polygons or lines that are on top of each other which could result in double-counting areas.
Esri Topology Rules in ArcGIS 10
Topology rules ensures connectivity between GIS datasets. It fills in gaps, removes slivers and fixes overlapping polygons. When polygons share a boundary, topology rules enforce data integrity.
…But you already know that.
Let’s get into the nuts and bolts of how to use topology in ArcGIS.Step 1: Add your data to a geodatabase
Creating topology rules in ArcGIS is just a couple of simple steps. First, add your data in a feature data set.
Step 2: Create topology data set
Once your data is in a geodatabase, you have to create a topology data set.
Follow the instructions by selecting your feature class.Step 3: Add topology rules
There are over 30 rules you can add to inspect the quality of your data. One-by-one, you add rules which govern exactly how to perform quality control to your spatial data. Below is a screenshot of adding an overlap rule.
Overlapping polygons can be resolved by the topology rule “must not overlap”.
Select yes to validate. This is how ArcGIS refreshes and ensures that it’s using the most recent data errors. The geodatabase should have a topology dataset within it now:
Step 4: Inspect your data
Once your data is in a geodatabase and rules have been added, you can now inspect your data topology. ArcGIS topology fixing is interactive. One-by-one, you can go through errors and fix them with the error inspector.
Click the Add Data button
Add the topology data set.
The errors in your data will appear as area, line and point errors.
Step 5: Correct your errors
The ArcGIS topology tools supports richer, complex error checks. Resolve topology issues with automatic or manual fixes.
Add the Topology Toolbar by clicking Customize > Toolbars > Topology. If the Topology Toolbar is greyed out, you have to turn on Editing mode by clicking Editor > Start Editing.
When you push the error inspector button, you can manually go through each individual error.
Going through the error inspector report, you can right-click individual errors for quick fixes. Here are the types of topology fixes you can perform:
- Merging: The overlapping portion will be subtracted from one polygon and added to the other polygon.
- Subtract: The overlapping portion will be removed completely from both polygons.
- Create Feature: The overlapping portion will be removed completely from both polygons and added as an entirely new features.
Preventing Errors by Using the Snapping Environment
Using snapping in GIS can prevent errors from happening altogether. The snapping environment determines the distance that newly digitized points, lines and polygons will occupy the same location of existing features.
When new features are created in the editor, they will be automatically snapped to existing features. For example, you can snap to edges, points, ends, intersections, midpoints and tangents.
When errors exist, topology tools can help… And clean topology is absolutely necessary in GIS.
Topology is especially useful for detecting and correcting errors of digitized data because it checks and validates the spatial relationship of neighboring and overlapping features.
Topology editing is a strong point in ArcGIS with a ton of options to fix editing errors.
It’s really the best of its kind.