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| GeoCommunity Mailing List |
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| Subject: | RE: GISList: projection of maps |
| Date: |
10/07/2002 07:15:12 AM |
| From: |
Fass, Jim |
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Jayachandran,
You are correct to doubt that you need to project data from geographic
coordinates, since your source is already in your Indian projection. The
real problem is that your projected coordinates are not listed for the
registration corners of your source map, so you don't have good coordinates
to start digitizing from. What I recommend is that you prepare a map
neatline in decimal degrees (lat/lon) that corresponds with the extent of
your source map. This should look like a perfectly orthogonal rectangle.
If possible, densify the vertices of the triangle so you have some several
coordinates along each of the sides. Then project your lat/lon rectangle to
the parameters of your source map. Now the rectangle should have the
characteristic shape of the neatline of your map--probably some curvature
along the north and south neatlines, and some grid declination. At this
point, you can read the coordinates of the neatline corners. These will be
the coordinates you want to use to set up and digitize your map, already
projected.
Now, a word about what happens if you try it the other way (as described in
your post)...
If you start digitizing in lat/lon while the shape of your map is already
projected, then any curvature along lines of latitude with be a cause for
distortion in your collected coordinates. This distortion will be amplified
if/when you apply another projection. In general, when digitizing from
paper sources, always prepare a target neatline for registration that
matches the projection parameters of the source map, then digitize to that
target.
Hope this helps.
--Jim
James C. Fass
Program Director
Analytical Surveys, Inc.
11900 Crownpoint Dr., Suite 100
San Antonio, TX 78233
(210) 657-1500 x215
FAX (210) 657-1304
jfass@anlt.com
-----Original Message-----
From: Jayachandran Mani [mailto:jayachandran_m@yahoo.com]
Sent: Sunday, October 06, 2002 10:19 PM
To: gislist@geocomm.com
Subject: GISList: projection of maps
Hi listers,
I need some clarifications on the "projection" process
used by more people using the survey of India
toposheets for digitisation in India. It may be more
regional. I apologise for posting this in the list.
But I believe i will be getting clarified of my doubt
here.
SOI maps are normally in polyconic projection with
everest spheroid. But the maps will have only the
lat/lon values on it. Those who digitise the SOI maps
use the intersection of lat/lon grids as their TIC
points for projecting into real ground values. Here
the information provided to the GIS software is as
follows
input projection - geographical
output projection required - polyconic
spheroid - Everest
units - meters
central meridian - the central meridian value of the
project area
Is it correct. I understand geographical means the
real lat/lon value of the point. But as the map is
already in paper format, the source itself is a
projected (polyconic) map. then what is the use of
projecting a map which is already in the polyconic
projection again to a polyconic projection. It is
confusing for me. Is this process is only to get the
rectangular co-ordiantes alone.??.
Please clarify
Thanks for your support
Regards
Jay
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