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On 29/04/2011 13:22, Morin, Marc-André wrote:<br>
Hi Marc,<br>
<br>
Thanks for the advice, I had choosen to use 4326 mainly because its
the default in postgis for so many calls.<br>
<br>
I shall just convert my data to 900913/3857 and forget having to
convert anything on the fly.<br>
<blockquote
cite="mid:33E3342755CFC844901AD4572AD514A404B62042@lauqueex01.lau.dfo-mpo.ca"
type="cite">
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<div dir="ltr" align="left"><span class="278473211-29042011"><font
color="#0000ff" face="Arial" size="2">Hi Dave,</font></span></div>
<div dir="ltr" align="left"><span class="278473211-29042011"></span> </div>
<div dir="ltr" align="left"><span class="278473211-29042011"><font
color="#0000ff" face="Arial" size="2">You are certainly not
on the right track, because the SRS EPSG:4326 is not a
projection system, but more a datum (WGS84), based on an
ellipsoid (GRS80). In a nutshell, the 4326 srs
is a "spherical" geographic system and that is why the unit
of measurement is in degree. So when you store your data in
EPSG:4326, that means your data will have to be stored in
latitude and longitude, and you cannot represent lat-long
coordinates directly on a map, unless these coordinates are
projected in a cartesian (X and Y) coordinate system, like
the Google Mercator projection (EPSG:900913).</font></span></div>
<div dir="ltr" align="left"><span class="278473211-29042011"></span> </div>
<div dir="ltr" align="left"><span class="278473211-29042011"><font
color="#0000ff" face="Arial" size="2">That said, you
specified in your first mail that your were working with
"lines" measured in nautilcal miles, within a grid graduated
for 0 to 100. That means that you are already in
a "projected" coordinate system. Now, we have to figure
out what is this coordinate system. Maybe I am wrong, but
your 0-100 grid seems to represent a cartesian coordinate
system in GRAD (not in degree)... </font></span></div>
<div dir="ltr" align="left"><span class="278473211-29042011"></span><span
class="278473211-29042011"></span> </div>
<div><span class="278473211-29042011"></span><font face="Arial"><font
color="#0000ff"><font size="2">C<span
class="278473211-29042011">ould you check and
confirm that at first? Also, a screenshot of your map
would be helpful...</span></font></font></font></div>
<div><font face="Arial"><font color="#0000ff"><font size="2"><span
class="278473211-29042011"></span></font></font></font> </div>
<div><font face="Arial"><font color="#0000ff"><font size="2"><span
class="278473211-29042011">Regards,</span></font></font></font></div>
<div><font face="Arial"><font color="#0000ff"><font size="2"><span
class="278473211-29042011"></span></font></font></font> </div>
<div><font face="Arial"><font color="#0000ff"><font size="2"><span
class="278473211-29042011">Marc-André</span></font></font></font></div>
<div><font face="Arial"><font color="#0000ff"><font size="2"><span
class="278473211-29042011"></span></font></font></font> </div>
<div><br>
</div>
<div class="OutlookMessageHeader" dir="ltr" align="left" lang="fr">
<hr tabindex="-1">
<font face="Tahoma" size="2"><b>De :</b>
<a class="moz-txt-link-abbreviated" href="mailto:postgis-users-bounces@postgis.refractions.net">postgis-users-bounces@postgis.refractions.net</a>
[<a class="moz-txt-link-freetext" href="mailto:postgis-users-bounces@postgis.refractions.net">mailto:postgis-users-bounces@postgis.refractions.net</a>] <b>De
la part de</b> Dave<br>
<b>Envoyé :</b> 29 avril 2011 04:05<br>
<b>À :</b> PostGIS Users Discussion<br>
<b>Objet :</b> Re: [postgis-users] drawing nautical data on
projection 4326[solution?]<br>
</font><br>
</div>
I think I have a solution to this issue<br>
<br>
I am going to convert my point origin to meter based projection
(eg google 900913), create my line data, then transform it back to
the 4326 projection, in this way the scale factors calculations
should be done by the proj library.<br>
<br>
Does this sound like a decent solution?<br>
<br>
On 28/04/2011 10:48, Dave wrote:
<blockquote cite="mid:4DB937FE.2050007@pinan.co.uk" type="cite">I
have some line data measured in nautical miles.<br>
<br>
I want to draw these points on a map using the 4326 projection,
the srs entry for this projection gives the unit of measurement
as degree.<br>
<br>
A nautical mile is one minture of an arc of lattitude ie 0-60,
the projection is measured in 0-100.<br>
<br>
So does this mean that when I want to draw a line measured in
nautical miles, I have map 0-60 on to 0-100 grid , ie I have to
apply 0.6 factor ?<br>
<br>
So a line starting at lat 1.2 and ending at lat 2.7 becomes a
line 1.12 to 2.42 ?<br>
<br>
regards<br>
<br>
<br>
Dave.<br>
<br>
<br>
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