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by mastax
1346 days ago
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As an aside, I've been trying to learn basic GIS with PostGIS and QGIS and it's been quite frustrating. I had a dataset of roads which were broken up into short segments, which I wanted to merge back together based on a key. Theoretically that's a single simple operation, in practice it was getting hung up on something I couldn't understand and it took all afternoon. My usual practice of JIT doc reading wasn't working well, too many unfamiliar terms and missing fundamentals. If you have any recommendations for books or docs I'd love to hear them. |
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> a dataset of roads which were broken up into short segments, which I wanted to merge back together based on a key. Theoretically that's a single simple operation
PostGIS provides the ST_MakeLine aggregate function for this, but you need to write the query such that the GROUP BY query retains the correct order. Creating a new line segment out of many line segments effectively means breaking the lines into their constituent points and then creating a new linestring based on the points. For things like GPS data, you can order by timestamp. But for other cases? You've got to write your aggregate query carefully so that adjacent line segments are actually meant to be merged.