OGR2OGR Esri JSON to GeoJSON – Rings returning as null in geometry?

OGR2OGR Esri JSON to GeoJSON: Why Are Your Polygon Rings Going AWOL?

So, you’re wrestling with geospatial data, huh? If you’ve ever had the pleasure of converting Esri JSON to GeoJSON using OGR2OGR, you might have run into a real head-scratcher: polygon rings mysteriously disappearing, turning into null values. Trust me, you’re not alone! It’s a common problem, and frankly, it can be a real pain. But don’t worry, we’re going to figure this out.

OGR2OGR is like that trusty multi-tool you keep in your geospatial toolkit – incredibly versatile for shuffling data between formats. Esri JSON is how ArcGIS likes to store its geographic goodies, while GeoJSON is the open-source darling everyone loves for its simplicity and widespread support. Getting them to play nice together should be straightforward, but sometimes… it’s not.

The culprit behind those vanishing rings? It usually boils down to how Esri JSON handles polygons, especially those fancy ones with holes in them (think donut shapes, or islands in a lake). Esri JSON describes these polygons as a collection of rings: the outer ring defines the overall shape, and the inner rings carve out the holes.

Here’s the kicker: GeoJSON has some pretty strict rules about which way those rings have to wind. It’s all about the “right-hand rule” – exterior rings must go counterclockwise, and interior rings must go clockwise. If your Esri JSON data doesn’t follow this convention, or if OGR2OGR gets confused along the way, you end up with those dreaded null rings, and your polygons look… well, broken. I remember one time I spent hours debugging a map only to realize the winding order was off. Lesson learned!

Let’s break down the usual suspects and how to deal with them:

1. Winding Order Gone Wild:

This is the most likely reason. Esri sometimes bends the rules a bit, allowing for different winding orders. GeoJSON? Not so much.

• The Fix: Before you even think about OGR2OGR, take a peek at your Esri JSON. If the winding order is consistently wrong (say, all the outer rings are clockwise), you’ll need to fix it. Python to the rescue! Libraries like shapely or pyproj can help you flip those rings around. It’s like giving your polygons a good haircut.

2. OGR2OGR Being a Grumpy Old Man (Version Issues):

Older versions of OGR2OGR can be a bit… finicky. They might not handle complex geometries or unusual winding orders very well.

• The Fix: Update, update, update! Seriously, make sure you’re running the latest stable version of GDAL/OGR. It’s like giving your trusty multi-tool a much-needed upgrade. Use your system’s package manager or grab the latest version from the GDAL website. Trust me, it’s worth it.

3. Coordinate Systems Throwing a Tantrum:

Okay, this is less common, but still worth checking. If your Esri JSON and GeoJSON are in different coordinate systems, OGR2OGR has to do some math to convert them. If something goes wrong during that conversion, you can end up with messed-up geometries.

• The Fix: Be explicit! Tell OGR2OGR exactly what coordinate systems you’re using with the -s_srs (source) and -t_srs (target) options. Like this:

  bash

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