[Proj] Lat/long to utm easting error

[Noel Zinn (cc)]

Adam,

What you're expecting works only on the Central Meridian (CM) and not 
elsewhere due to convergence of the meridians.

Google "convergence of the meridians" for a fuller explanation.

Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

-----Original Message----- 
From: Adam Dershowitz
Sent: Wednesday, January 05, 2011 6:18 PM
To: proj at lists.maptools.org
Subject: [Proj] Lat/long to utm easting error

I have run into something that I don't understand, and I hope someone can 
help explain.
I was doing some conversions of points, and ran into a problem.  I finally 
narrowed it down to the following:

If I take two points, at the identical longitude, with a small change in 
latitude, and convert them each to UTM, I would expect nearly identical 
easting values.  But I am not seeing that.  For example:

cs2cs +proj=latlong +datum=WGS84 +to +proj=utm +datum=WGS84 +zone=17 -r

42d37'3.77503"N 83d59'35"W
254524.57 4722707.58 0.00
42d37'0.02186"N 83d59'35"W
254520.47 4722591.80 0.00


Above there is  ~4 m easting difference, with just a small (~120m) change in 
latitude.  Since these points are exactly North-south of each other, 
shouldn't they have the same easting, at least when they are this close to 
each other in latitude?

Thanks much for any explanations,

--Adam



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