geography ST_Segmentize returning straight line instead of curved

[robe]

This appears to be a new regression which I am not seeing in PostGIS 2.2.2 install.

When I tried this query in PostGIS 2.3.0 I'm getting a straight line instead of a curve.

SELECT ST_AsText(ST_Segmentize(
ST_MakeLine(ST_Point(-73.8893, 40.6975), ST_POINT(72.8667,19.1583))::geography, 10000000)::geometry);

In 2.2.2 I get this which gets curvier as I reduce the segmentize meters POSTGIS="2.2.2 r14797" GEOS="3.5.0-CAPI-1.9.0 r4090" PROJ="Rel. 4.9.1, 04 March 2015" GDAL="GDAL 2.0.2, released 2016/01/26" LIBXML="2.7.8" LIBJSON="0.12" RASTER

LINESTRING(-73.8893 40.6975,19.6313806536934 62.1086776667385,72.8667 19.1583)

In POSTGIS="2.3.0dev r14864" GEOS="3.5.0-CAPI-1.9.0 r4088" PROJ="Rel. 4.9.1, 04 March 2015" GDAL="GDAL 2.1.0dev, released 2015/99/99" LIBXML="2.7.8" LIBJSON="0.12" RASTER I get

LINESTRING(-73.8893 40.6975,-0.511300000000007 29.9279,72.8667 19.1583)

Look at snapshot for perspective.

[robe]

I took a closer look at this to see what changed in PostGIS 2.2 to 2.3 and most likely culprit to me seems like change in

ptarray_segmentize_sphere in lwgeodetic.c in PostGIS 2.2 there was a called to geog2cart which looks like it was taken out and replaced with something else in PostGIS 2.3.0

at r14706

[robe]

reopened this pull request - ​​https://github.com/postgis/postgis/pull/90. We'll have to rollback this change unless a revised pull request is submitted and preferably one that includes a test for the general case, so we catch this kind of thing in the future.

[hmercier]

Hi, I may be missing something, but I fail to understand why the segmentize in 2.2 should curve the line ...

[robe]

Because this is the geography segmentize not the geometry segmentize. It should follow the great circle pattern. geography segmentize has now been reduced to nothing but geometry segmentize.

If I go from New York to Mumbai and I segmentize my line enough even on a flat map it should not show me going thru Africa.

Here is a query way of looking at it. Note how the 2.3 answer is way longer than the original segment and 2.2 answer

SELECT ST_Length(ST_MakeLine(ST_Point(-73.8893, 40.6975), ST_POINT(72.8667,19.1583))::geography) As orig_len,
ST_Length('LINESTRING(-73.8893 40.6975,19.6313806536934 62.1086776667385,72.8667 19.1583)'::geography) AS len_22_answer,
ST_Length('LINESTRING(-73.8893 40.6975,-0.511300000000007 29.9279,72.8667 19.1583)'::geography) AS len_23_answer;

     orig_len     |  len_22_answer   |  len_23_answer
------------------+------------------+------------------
 12544025.7530776 | 12544040.5248272 | 13998927.2849106
(1 row)

[hmercier]

Oops, of course, my bad ! I am fixing again the segmentize function in order to fix this issue (and still have segments of constant length). I will send a new patch quickly. Thanks for the report.

[robe]

While you are at it, add a test for this case. We don't have a test to catch this common use case.

[robe]

In 15044:

Segmentize geography using equal length segments, (Hugo Mercier of Oslandia)
Closes #3613
Closes #3539
Closes ​https://github.com/postgis/postgis/pull/90