[OSRS-PROJ] Pseudo X/Y values, forward and reverse

Duncan Agnew agnew at bilby.ucsd.edu
Fri Jan 31 17:54:10 EST 2003

Dear Mr. Hersman:

	I vote for "this is all foolishness". I am a geophysicist--not the
exploration kind, but close enough to know what is going on. What the software
wants is coordinates that it can use to get accurate shot-to-receiver
distances--and these distances are never that large. (Because if they were,
the seismic waves would be going too deep to drill, and your company wouldn't
care). And, the software wants to work on a flat Earth, so it uses a grid.

	But, unless the grid is tailored to the line, you can't get good
local accuracy over long distances (e.g., if your line ran NS you could
use a TM with a central meridian along that line). If you try to use (say)
SPCS Alaska Zone 4, things are going to get pretty distorted out to the west.

	Your choices are:

	1. Use something (lat&long) that will work over long distances. I
suspect this is ruled out by the software, and anyway you don't need it: you
only need accurate distances from X&Y over small distances. (Up to 50 km,

	2. Design a projection (grid) that is tailored to each line so that the
local accuracy will be high everywhere. For this you need an expert (Cliff);
and it may not be that easy if the seismic line is not linear.

	3. Break the analysis into smaller regions, and use the appropriate
SPCS for each region.

	I'd vote for 3: but remember, free advice is worth what you paid.

Duncan Agnew
dagnew at ucsd.edu

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