projection questions

[Jeff Portwine]

Thanks for the explanation, that does make perfect sense.    I believe all
of my shapefiles are NAD83... i'm not sure if they are all geographic but my
guess is that they are.    So I guess my only question is why my map looks
so ... skewed I guess is the only word I can come up with at the moment.  It
just doesn't look right... the whole map seems almost twisted so that the
coastline appears to be running more east/west than it should rather than
north/south.  Perhaps my "image size" is set wrong so that it's compressing
one axis and stretching another.... I will just have to play with it I
guess.

Thanks for the great information.   If nothing else this mapping project has
been educational.

-Jeff

----- Original Message -----
From: "Ed McNierney" <ed at topozone.com>
To: "Jeff Portwine" <jdport at veritime.com>; <MAPSERVER-USERS at LISTS.UMN.EDU>
Sent: Friday, January 21, 2005 12:56 PM
Subject: RE: [UMN_MAPSERVER-USERS] projection questions


Jeff -

A datum is not a projection, but a mathematical model of the Earth.  Every
geographic (i.e. latitude/longitude) and projected coordinate system is
expressed relative to a datum.  Many people incorrectly think that the
latitude and longitude of a point are absolute, unchangeable values.
Latitude and longitude are geographic coordinates expressed relative to a
datum.  If you change the datum, the coordinates will change.  In the United
States, the NAD27 and NAD83 datums are commonly used.  WGS84 is also used
because it is a world-wide datum, but in the USA you can consider it
identical to NAD83.

For example, I'm typing this from a location that's latitude 42.5860°N,
71.5553°W in WGS84/NAD83 coordinates, and 42.5859°N, 71.5558°W in NAD27
coordinates.  Not a big difference, but a real one.

If ALL of your shapefiles are geographic NAD83, things are pretty easy.  If
your input and output projections are all the same (and let's start with
that case) then you can completely ignore projections.  Just be sure the
extents in your .mapfile are appropriate for the area and the UNITS are DD
(decimal degrees).

It sounded, however, from your last email, as if you'd already had that
working.  What works and doesn't work?  Can you get one shapefile to display
using latitude/longitude extents for the northeastern US?

     - Ed