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| GeoCommunity Mailing List |
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| Subject: | [gislist] finding the y of a latitude for a mercator projection |
| Date: |
03/04/2004 09:50:01 AM |
| From: |
Tor Kristensen |
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Hi all,=20
=20
I'm developing a set of GIS libraries for flash, and have run into
a problem.=20
=20
Using the formula from :=20
http://mathworld.wolfram.com/MercatorProjection.html
log( (1.0 +sin(L) / (1.0 -sin(L) )/2.0: =20
=20
and the Gudermannian function:
http://mathworld.wolfram.com/GudermannianFunction.html
(2 * atan( sin(L) / cos(.5*L) ) )
=20
=20
I've succeeded in getting lat/lon correctly placed on a world map. (yay
me)
=20
However, when I am plotting points on a map which is not a world map, I
cannot figure out how to scale the gudermannian values so that they are
correctly positioned on the map.=20
=20
Essentially when I have a map whose latitude boundings are 70N 30N, I
know that it is 300pixels tall. However, I cannot calculate the height
of the total map from this, which is needed to scale the transformed lat
positions.
(As far as I can tell)
=20
=20
Essentially
V1 =3D gudermannian(degreesToRadians(-p3.latitude)):
Y =3D (V1*WorldHeightInPixels)+offsetToMapTop:
=20
I can calculate offsetToMapTop if I can figure out=20
WorldHeightInPixels.
=20
My rusty trig is suggesting I've missed something pathetically obvious
here.
=20
Any suggestions, algorithms, and tips are greatly appreciated.
Thanks in advance
Tor Kristensen
=20
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