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<div class="moz-cite-prefix">Things behave as you'd expect going
north/south because meridians of longitude are great circles
(shortest paths on globe).<br>
However, when you go east/west, parallels of latitude and great
circles (shortest paths) are <b>not</b> the same. Parallels are
convenient lines representing the graticule and represent <i>orthodromes</i>
(lines of constant azimuth) but are not true great circles (which,
presumably, st_project is following).<br>
(FYI, if you were to follow an orthodrome in any other azimuth but
the four cardinal directions, you'd spiral towards a pole!)<br>
Spherical trigonometry's not always simple ;-)<br>
<pre class="moz-signature" cols="72">Martin Feuchtwanger <a class="moz-txt-link-abbreviated" href="mailto:feumar@shaw.ca">feumar@shaw.ca</a> 604-254-0361
Vancouver, BC V5N 1J6
<a class="moz-txt-link-freetext" href="http://members.shaw.ca/geomatics.developer">http://members.shaw.ca/geomatics.developer</a>
</pre>
On 21/02/2013 12:15 AM, News and Such wrote:<br>
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<div class="post-text" itemprop="description"> Hello postgis'ers,<br>
<br>
somehow I can't understand the results of ST_Project, but maybe
this problem is just me being stupid (again):<br>
I just want to project a point in North, East, South and West
direction.<br>
<br>
Note:<br>
I use geography-type here, but the results are the same for
geometry.<br>
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.<br>
<br>
With the direction north, everything is as I expect it:<br>
<br>
<code>SELECT ST_AsText(ST_Project('POINT(-30 -0.5)'::geography,
1e7, radians(0))); <br>
st_astext ----------------------------- <br>
POINT(-30 89.4874128664049) </code><br>
<br>
But in the other directions, here for example East, I get this:<br>
<br>
<code>SELECT ST_AsText(ST_Project('POINT(-30 -0.5)'::geography,
1e7, radians(90))); <br>
st_astext --------------------------------------------- <br>
POINT(59.8315233716443 0.00116689648502556) </code><br>
<br>
Shouldn't this be <code>POINT(59.8315233716443 -0.5)</code> ?
Why are both coordinates changing?<br>
<br>
This behaviour occurs in all directions save north. <br>
<br>
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.<br>
<br>
Any help would be appreciated! <br>
Thank you.<br>
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</pre>
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