[postgis-users] not so Weird behaviour of ST_Project (PostGIS2)

[News and Such]

I was afraid that this was just me being stupid again.
Thanks so much for the explanation!


On 02/22/2013 07:57 PM, Martin Feuchtwanger wrote:
> Things behave as you'd expect going north/south because meridians of 
> longitude are great circles (shortest paths on globe).
> However, when you go east/west, parallels of latitude and great 
> circles (shortest paths) are *not* the same. Parallels are convenient 
> lines representing the graticule and represent /orthodromes/ (lines of 
> constant azimuth) but are not true great circles (which, presumably, 
> st_project is following).
> (FYI, if you were to follow an orthodrome in any other azimuth but the 
> four cardinal directions, you'd spiral towards a pole!)
> Spherical trigonometry's not always simple ;-)
> Martin Feuchtwangerfeumar at shaw.ca   604-254-0361
> Vancouver, BC  V5N 1J6
> http://members.shaw.ca/geomatics.developer
> On 21/02/2013 12:15 AM, News and Such wrote:
>> Hello postgis'ers,
>>
>> somehow I can't understand the results of ST_Project, but maybe this 
>> problem is just me being stupid (again):
>> I just want to project a point in North, East, South and West direction.
>>
>> Note:
>> I use geography-type here, but the results are the same for geometry.
>> I use a rather long distance, because I need it, at least in one 
>> direction, for my application. Because of this I found the "problem" 
>> in the first place.
>>
>> With the direction north, everything is as I expect it:
>>
>> |SELECT ST_AsText(ST_Project('POINT(-30 -0.5)'::geography, 1e7, 
>> radians(0)));
>> st_astext -----------------------------
>> POINT(-30 89.4874128664049) |
>>
>> But in the other directions, here for example East, I get this:
>>
>> |SELECT ST_AsText(ST_Project('POINT(-30 -0.5)'::geography, 1e7, 
>> radians(90)));
>> st_astext ---------------------------------------------
>> POINT(59.8315233716443 0.00116689648502556) |
>>
>> Shouldn't this be |POINT(59.8315233716443 -0.5)| ? Why are both 
>> coordinates changing?
>>
>> This behaviour occurs in all directions save north.
>>
>> I'm new to the whole earth coordinate system-stuff so maybe I missed 
>> something, but I thought that when one walks directly west (or south) 
>> one would, by definition, not leave the current longitude (or 
>> latitude)-line but only change the other.
>>
>> Any help would be appreciated!
>> Thank you.
>>
>>
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