[postgis-users] ST_Distance provides unexpected results when using Geography type

[Wayne Rowcliffe]

Wow. That is not at all what I expected.

Thanks for the handy tip.

*Wayne Rowcliffe*
Software Engineer, Priority5 Holdings
E-mail: wayne at priority5.com




On Thu, Mar 5, 2020 at 5:06 PM Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

> If you would like to see what your “boxes” actually look like, try
> wrapping them in ST_Segmentize() before viewing them on a flat map.
>
> select st_asgeojson(st_segmentize('SRID=4326;POLYGON ((-167.51953125
> 8.841651120809145, 1.23046875 8.841651120809145,
> 1.23046875 63.35212928507874, -167.51953125 63.35212928507874,
> -167.51953125 8.841651120809145))'::geography,500000))
>
> Past that into GeoJSON.io <http://geojson.io/> and see what it looks
> like. All will become clear.
>
> P
>
>
>
> On Mar 5, 2020, at 2:41 PM, Wayne Rowcliffe <wayne at priority5.com> wrote:
>
> Hi,
>
> I've attached a file containing sample queries that reproduce what I am
> seeing.
>
> Basically, we have an application that shows data on the globe. We are
> filtering the results to those within the current window viewport. What
> I am seeing is that depending on where the map is, I'll either get results
> or I won't, even when there should be data visible.
>
> While investigating, I switched to calling ST_Distance and noticed that
> when using geometry the results were always accurate. However,
> when using geography (like we do in our application), we will sometimes get
> the correct result, and other times it will say a point within the
> bounding geometry is actually hundreds of thousands of meters away.
>
> This is my version string using the query on the postgis webpage:
> PostgreSQL 10.9 (Ubuntu 10.9-1.pgdg16.04+1) on x86_64-pc-linux-gnu,
> compiled by gcc (Ubuntu 5.4.0-6ubuntu1~16.04.11) 5.4.0
> 20160609, 64-bit POSTGIS="2.5.2 r17328" [EXTENSION] PGSQL="100"
> GEOS="3.5.0-CAPI-1.9.0 r4084" PROJ="Rel. 4.9.2, 08 September
> 2015" GDAL="GDAL 1.11.3, released 2015/09/16" LIBXML="2.9.3"
> LIBJSON="0.11.99" LIBPROTOBUF="1.2.1" TOPOLOGY RASTER
>
> The closest bug I could find was this:
> https://trac.osgeo.org/postgis/ticket/4480
> It seems potentially different though since the bounding area of my query
> is less than 180 degrees.
>
> Wayne Rowcliffe
> Software Engineer, Priority5 Holdings
> E-mail: wayne at priority5.com <wayne at priority5.com>
>
>
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