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Correction...that url was misspelled - it's supposed to be
<a class="moz-txt-link-freetext" href="http://www.epsg.org">http://www.epsg.org</a> - sorry.<br>
<br>
Mike Leahy wrote:
<blockquote cite="mid42FA2A0F.2090101@fes.uwaterloo.ca" type="cite">
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Hey there,<br>
<br>
I usually use transform() to convert my data to a UTM projection, so
that I can calculate distances in metres with relative ease - maybe
there's a more accurate way, but this works well enough for me.<br>
<br>
As for looking up SRID numbers, someone (either in this list or
another) directed me to <a class="moz-txt-link-freetext"
href="http://www.espg.org">http://www.espg.org</a>, where you can
download a
database of EPSG projection information, including a version in
Microsoft Access format with forms for browsing. I believe the EPSG
identifier is usually the same as the SRID in the spatial_ref_sys table
included with PostGIS (but you'll want to compare the records for
whatever projection you need to make sure I'm right about that).<br>
<br>
Regards,<br>
Mike<br>
<br>
Lance Arlaus wrote:
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<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">I know this is a simple
question, but I’m just getting
started with PostGIS and I can’t seem to perform a simple calculation.</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">I just need to calculate
the distance between two
points. What am I doing wrong?</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Query:</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> SELECT
distance(PointFromText('POINT(40.8117 -74.218)', 32617),
PointFromText('POINT(40.8272 -74.1242)', 32617))</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Returns:</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> 0.095…</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Of course if I do the
following, I get a reasonable answer
(about 10km), but it’s not exactly what I want</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> SELECT
distance_sphere(PointFromText('POINT(40.8117 -74.218)', 32617),
PointFromText('POINT(40.8272 -74.1242)', 32617))</span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Even
constructing a line and taking the
length doesn’t work and I’ve tried all SRIDs.</span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Sorry to ask such a
simple question.</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Also, is there a document
that describes the well known
SRIDs? I’m not sure which one to use. For example, the data
I’ve got references WGS 84, but there’s a ton of SRIDs
defined. I’ve found docs describing the format of the
specification, but how are the SRIDs created (who serves as the
clearinghouse
for this info)?</span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;"> </span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial;">Thanks.</span></font></p>
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