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0 pointsStratoscope1d ago0 comments

One task where GeoJSON falls down is simplification of a group of polygons with common boundaries, e.g. the 48 conterminous US states. If you start with a highly detailed set of polygons, you need to simplify them for practical display in an online map.

GeoJSON doesn't encode the fact that the boundary points are common between adjacent polygons. When you simplify those polygons, each one is handled separately, and you end up with "slivers" where the boundaries are misaligned:

https://www.bing.com/images/search?q=map+slivers+betwen+poly...

TopoJSON solves this by encoding each such boundary only once. So when you simplify the polygons, they are all done together, and the same simplification applies to adjacent polygons. No more slivers!

https://github.com/topojson/topojson

https://github.com/topojson/topojson-simplify

0 comments

Demiurge1d ago

Is this actually GeoJSON falling down, or decades of convention extended to JSON? Topology is great, but it is sidestepped by Shapefile/WKT/WKB/etc, in favor of independent primitives like POINT, LINE, POLYGON. If GeoJSON did not exist as a new JSON GIS data format encoding these primitives, TopoJSON would not have "replaced" it, due to the added mis-match with other non-topological formats.

From what I can tell, the top criticism of GeoJSON is the under-enforced winding order specification, and crossing the antemeridian.

jvanderbot1d ago

Right. Encoding a union algorithm into the data structure just introduces the reverse problem: Selecting a subset now requires extra logic beyond jq.

StratoscopeOP1d ago

Similarly, typical map APIs like the Google Maps API accept GeoJSON and not TopoJSON. I was not suggesting TopoJSON as a replacement for GeoJSON, but as a complement to it. With the tools on the TopoJSON GitHub, you can have GeoJSON input and output, but convert to TopoJSON for the simplification step to avoid the "slivers" problem.

pramsey1d ago

GeoJSON is not TopoJSON. Saying that is "falling down" is like criticizing a zebra for not being a giraffe. GeoJSON is a mapping of the (non-topological) "simple features" model into JSON, full stop. It does that fine.

StratoscopeOP1d ago

Yes, the same "slivers" problem occurs when you try to simplify features in any format that uses individual polygons, such as shapefiles or whatnot. That's the only case I was referring to.

I don't think I would trust a zebra or a giraffe for this task either.

echoangle1d ago

How is that a geojson problem? If your dataset is correct, adjacent borders will just use the same points and will match exactly.

sdenton41d ago

The problem is simplification. Suppose two regions share a border with some nonlinear points a, b, c, d. Simplifying the polygon for the first region might yield a, b, d while the second yield a, c, d. This creates gaps or overlaps between the two regions.

qurren1d ago

But what is the border? Set the border to what it actually is, not a simplification of it. The state of Colorado is formally a 697 sided polygon, don't simplify it to a rectangle.

3 more replies

echoangle1d ago

So don’t simplify the shapes on their own. Geojson is a storage and exchange format, you can still convert it to other formats if you want to modify it.

1 more reply

NelsonMinar1d ago

I like TopoJSON and have used it in projects. But it's weird to set it up as opposition to GeoJSON. It's a complement. GeoJSON is a general data format meant to replace uses of ESRI Shapefiles and other complex formats. TopoJSON is more of a solution for a particular application need.

Is there much work developing or using TopoJSON these days? I haven't seen much about it in a few years.

StratoscopeOP1d ago

To be clear, I'm not suggesting TopoJSON as an alternative to GeoJSON. I like GeoJSON and was loosely involved with the working group that created and updated its spec.

I'm just saying that for the specific task I mentioned GeoJSON or any format such as shapefiles that store polygons individually naturally leads to the "sliver" problem.

A nice processing pipeline is:

1. Convert GeoJSON to TopoJSON.

2. Run the simplification on the TopoJSON.

3. Convert the resulting TopoJSON back to GeoJSON.

The TopoJSON GitHub has tools for each of these steps.

j / k navigate · click thread line to collapse

0 comments

Demiurge1d ago

From what I can tell, the top criticism of GeoJSON is the under-enforced winding order specification, and crossing the antemeridian.

jvanderbot1d ago

Right. Encoding a union algorithm into the data structure just introduces the reverse problem: Selecting a subset now requires extra logic beyond jq.

StratoscopeOP1d ago

pramsey1d ago

StratoscopeOP1d ago

Yes, the same "slivers" problem occurs when you try to simplify features in any format that uses individual polygons, such as shapefiles or whatnot. That's the only case I was referring to.

I don't think I would trust a zebra or a giraffe for this task either.

echoangle1d ago

How is that a geojson problem? If your dataset is correct, adjacent borders will just use the same points and will match exactly.

sdenton41d ago

qurren1d ago

But what is the border? Set the border to what it actually is, not a simplification of it. The state of Colorado is formally a 697 sided polygon, don't simplify it to a rectangle.

3 more replies

echoangle1d ago

So don’t simplify the shapes on their own. Geojson is a storage and exchange format, you can still convert it to other formats if you want to modify it.

1 more reply

NelsonMinar1d ago

Is there much work developing or using TopoJSON these days? I haven't seen much about it in a few years.

StratoscopeOP1d ago

To be clear, I'm not suggesting TopoJSON as an alternative to GeoJSON. I like GeoJSON and was loosely involved with the working group that created and updated its spec.

I'm just saying that for the specific task I mentioned GeoJSON or any format such as shapefiles that store polygons individually naturally leads to the "sliver" problem.

A nice processing pipeline is:

1. Convert GeoJSON to TopoJSON.

2. Run the simplification on the TopoJSON.

3. Convert the resulting TopoJSON back to GeoJSON.

The TopoJSON GitHub has tools for each of these steps.

j / k navigate · click thread line to collapse