<html><head><style>body{font-family:Helvetica,Arial;font-size:13px}</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Brent,</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Had me going there for a sec, but Phew. It’s not me, it’s you.</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Ok, here’s my thought experiment, and please forgive me for doing it in the northern hemisphere.</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">- I start at YVR and point my plane directly east, and start flying a great circle route, such that if I go on in a straight line, I’ll eventually hit YVR again. After 200 km am I (a) south (b) north (c) directly east of YVR?</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Here’s one thing I know for sure, it’s not (c), because a line of latitude (except for the equator) is not a great circle. (It’s (a).)</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Or, let’s do another one.</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">- You stand here at latitude 45. I’ll stand 200 miles to the east, on latitude 45. What bearing will you take to get to me, using the shortest possible path?</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="margin: 0px;">select degrees(ST_Azimuth('POINT(-126 45)'::geography, 'POINT(-125 45)'::geography));</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">Seeing it? Stand further away.</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">select degrees(ST_Azimuth('POINT(-126 45)'::geography, 'POINT(-25 45)'::geography));</div> <div id="bloop_sign_1387231571592606976" class="bloop_sign"><div><br></div><div>The world is not flat and flat reasoning will lead you to the wrong places.</div><div><br></div><div>In your case, going directly east one way and then directly west the other, should lead to a result to the north of where you started, (a zig-zaggin path) as indeed it does.</div><div><br></div><div>P.</div><div><br></div><span style="font-family:helvetica,arial;font-size:13px"></span>-- <br>Paul Ramsey<br>http://cleverelephant.ca<div>http://postgis.net</div></div> <br><p style="color:#A0A0A8;">On December 16, 2013 at 2:00:18 PM, Brent Wood (<a href="mailto://pcreso@pcreso.com">pcreso@pcreso.com</a>) wrote:</p> <blockquote type="cite" class="clean_bq"><span><div><div>
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<div>Hi,</div>
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<div style="color:rgb(0, 0, 0);font-size:16px;font-family:times new roman, new york, times, serif;background-color:transparent;font-style:normal;">
I have been looking at using ST_Project() to solve a problem I have
(& in theory it is perfect!).</div>
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It seems to introduce some inaccuarcies, which can be quite
significant in some use cases. The SQL below projects a new point
2km E, then back 2km W. The result should be in the same place we
started, but is not, by a larger value than I'm comfortable with.
The source point location comes from a real dataset.<br></div>
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<span style="font-family:Courier New, courier, monaco, monospace, sans-serif;">select
Postgis_full_version();<br>
<br>
POSTGIS="2.0.1 r9979" GEOS="3.3.8-CAPI-1.7.8" PROJ="Rel. 4.8.0, 6
March 2012" GDAL="GDAL 1.9.2, released 2012/10/08" LIBXML="2.7.8"
TOPOLOGY RASTER<br></span><br></div>
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<br></div>
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<span style="font-family:Courier New, courier, monaco, monospace, sans-serif;">select
ST_AsText(point) as point,<br>
ST_AsText(ST_Project(point,
2000,radians(90))) as proj2k,</span></div>
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<span style="font-family:Courier New, courier, monaco, monospace, sans-serif;">
ST_AsText(ST_Project(ST_Project(point,
2000,radians(90)), 2000,radians(270))) as proj2k0<br>
from (select ST_AsText(ST_SetSRID(ST_MakePoint(171.0259489718,
-45.2961933568),4326))::geography as point) as foo;</span></div>
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<span style="font-family:Courier New, courier, monaco, monospace, sans-serif;"><br>
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<span style="font-family:Courier New, courier, monaco, monospace, sans-serif;">point
| POINT(171.0259489718 -45.2961933568)<br>
proj2k | POINT(171.051446314948 -45.2961905108324)<br>
proj2k0 | POINT(171.025948973075 -45.296187664865)<br>
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The final lat & lon should be the same as the original, but
both have changed, the latitude substantially more than the
longitude - even though the point latitude should be unchanged (it
only moved E & W, not N or S) </div>
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<div style="color:rgb(0, 0, 0);font-size:16px;background-color:transparent;font-style:normal;font-family:times new roman, new york, times, serif;">
It gets worse with larger distances, eg, at 2000km:</div>
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<br></div>
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<span style="font-family:Courier New, courier, monaco, monospace, sans-serif;">select
ST_AsText(point) as point,<br>
ST_AsText(ST_Project(point,
2000000,radians(90))) as proj2000k,<br>
ST_AsText(ST_Project(ST_Project(point, 2000000,radians(90)),
2000000,radians(270))) as proj2000k0<br>
from (select ST_AsText(ST_SetSRID(ST_MakePoint(171.0259489718,
-45.2961933568),4326))::geography as point) as foo;<br>
<br>
point | POINT(171.0259489718
-45.2961933568)<br>
proj2000k | POINT(-164.264848338683 -42.5385273590618)<br>
proj2000k0 | POINT(172.015779821441 -40.0225960863261)</span></div>
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<br></div>
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<span style="font-family:times new roman, new york, times, serif;">it causes
over 5 degree change in the latitude.</span></div>
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<br></div>
<div style="color:rgb(0, 0, 0);font-size:16px; background-color:transparent;font-style:normal;font-family:times new roman, new york, times, serif;">
Is this a bug I should be submitting or have I missed
something?</div>
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Thanks</div>
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<br></div>
<div style="color:rgb(0, 0, 0);font-size:16px;background-color:transparent;font-style:normal;font-family:times new roman, new york, times, serif;">
Brent Wood<br></div>
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<br></div>
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