[postgis-users] PostGIS Geography and Rhumb Lines

Derek Meyer derek at meyerprofessional.com
Wed Nov 28 11:44:23 PST 2012

Outstanding, thanks guys!

So am I correct that with something like the below I'd have a
resulting geometry of coords out of a 2D plain, correct?  Assuming I
got an acceptable SRID for Mercator there (3857?).

ST_Segmentize(ST_GeomFromText('LINESTRING(-29 -27,-30 -29.7,-36
-31,-45 -33)'), 3857), 5)

Then when worked into a query that bundles everything up via
ST_GeographyFromText, that each tiny segment would be technically
great-circle again, but *overall* a reasonable rhumb line
representation cast by the previous segmentize?

Makes sense to me.  My only other question would be probably a newbie
syntactical question regarding how to get the above segmentize result
within the SQL query.  Would I be using ST_AsText to return it to WKT
and concating it somehow within my ST_GeographyFromText line?  I'm
sure the below would probably break but, to see what I'm getting at...

ST_GeographyFromText('GEOMETRYCOLLECTION(LINESTRING(-28 -25, -27.6
-25.5), ' .ST_AsText(ST_Segmentize(ST_GeomFromText('LINESTRING(-29
-27,-30 -29.7,-36 -31,-45 -33)'), 3857), 5)). ')');

Just struggling to find an example online.  But given that, I'm just
about there.  Thanks again!

- Derek

On Wed, Nov 28, 2012 at 8:47 AM, Paul Ramsey <pramsey at opengeo.org> wrote:
> ST_Segmentize in geography will just segmentize along the great
> circle. You'll have to choose whether you want your model in geometry
> or geography. If geometry, then you can use Mercator as your
> projection (rhumb lines are then straight line) and use
> ST_Segmentize(geography) to convert your great circles into
> approximations in mercator.
> Or, you can use geography, and leave your great circle lines alone,
> but project your rhumb lines into geometry in mercator, st_segmentize
> them *there*, then convert htem back to geography.
> Clear as mud?
> P.
> On Wed, Nov 28, 2012 at 4:33 AM, Sandro Santilli <strk at keybit.net> wrote:
>> PostGIS-2.1 (in development) has an ST_Segmentize implementation for
>> "geography" type, which should help with rhumb lines.
>> You may or may not need to segmentize depending on the operation you
>> want to do on those lines. Anyway it would be practical to use in your
>> SQL queries if you don't want to store the segmentization persistently.
>> --strk;
>> On Wed, Nov 28, 2012 at 01:39:09AM -0800, Derek Meyer wrote:
>>> My current project involves creating a spatial database of world-wide
>>> airspace data, where each known flight information region is a polygon
>>> containing boundaries that are a combination of great circle lines,
>>> rhumb lines, and arcs.
>>> It seems straight forward enough that these polys could be easily
>>> inserted into the database via ST_GeographyFromText fed with a WKT
>>> GEOMETRYCOLLECTION of these compound boundary types, but I'm wondering
>>> about how to deal with rhumb lines in such a case since the Geography
>>> functions, if I understand correctly, base everything around a round
>>> earth model and utilize great-circle by nature.
>>> Is there any way to explicitly declare a rhumb line in an insert query
>>> such as this?  I've seen some examples elsewhere where a temporary
>>> projection to 2D is utilized and then segmentized to
>>> render/interpolate coordinates for a linestring that would more
>>> accurately represent said rhumb line.  Is this still the accepted
>>> approach?  Practical?  If so, how would such an approach be
>>> implemented in context of the SQL query?
>>> Any info/suggestions welcome and appreciated.
>>> Thank you for your time,
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