projection questions

[Ed McNierney]

Jeff -

You're confusing this New Englander with your definition of "New England" <g>.  It doesn't include Pennsylvania, Pennsylvania doesn't have much of a coast, and the coast from New York City to Eastport, Maine runs 4.2 degrees north and 7.0 degrees east, which is quite a bit closer to east than north, particularly in a lat/lon projection!

I think you're being misled by your MapPoint map.  I'm going to guess that it uses something like an Albers projection centered on the continent.  This will cause the eastern portions of the US to appear to rotate counterclockwise.

As a simple check, the northern borders of Pennsylvania, Connecticut, Rhode Island, and Massachusetts are all generally straight lines running east-west at constant latitude.  Are they running straight right to left on your Map Point map, or are they running at an angle?

Stretching the extents will not reproject the image.  If your MapPoint map doesn't have the Connecticut-Massachusetts border as a right-left line, no amount of X and Y stretching will rotate it into the place you expect.

If you want to duplicate the MapPoint map, the first thing you need to do is find out what projection it uses.  It sounds like it is NOT the projection you think it is, and I don't think it's very productive to try to guess at it.

     - Ed

P.S. Please remember to always send replies to the whole list - thanks!

Ed McNierney
TopoZone.com

-----Original Message-----
From: Jeff Portwine [mailto:jdport at veritime.com] 
Sent: Monday, January 24, 2005 10:16 AM
To: Ed McNierney
Subject: Re: [UMN_MAPSERVER-USERS] projection questions


> Have you looked at the coordinate values in your files?  Are they 
> plausible geographic
> (latitude/longitude) numbers?

Yes, they do appear to be geographic lat/long numbers, and all of my data 
sets seem to use the NAD83 datum.

> But I'm a bit puzzled about your coastline comment - there is very little 
> "north/south"
> coastline in the northeastern US.  It runs east/west or 
> northeast/southwest most of the time.

Basically, the coastline from Pennsylvania up to Maine runs roughly to the 
northeast, but more north-south than east-west.    My map via mapserver 
appears to be more east west.

It's possible that microsoft mappoint just prints out screwy maps... but I'm 
trying to duplicate this map that was sent to me that was created in 
mappointe and I just can't seem to do it.   I tried to set the extents for 
my output to match what the extents appear to be in the mappoint map 
(looking at cities that are close to the borders of the map and using the 
lat/long of those cities as my extents).   I would think that mapserver 
would generate an image that would stretch or compress to fill the "image 
size" and I figured that would make the maps look alike (if I created an 
image size that was the same as or proportional to the map point map image 
size).   It doesn't seem to do that though, regardless of what I set the Y 
extents to , it shows the same range, just shifts the map north or south.

I am not sure if i'm expressing myself right, I hope you understand what I'm 
trying to do and what I'm actually seeing.    Perhaps I could send you 
images if you don't understand what I mean.

Thanks,
Jeff


- Ed

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On 
Behalf Of Jeff Portwine
Sent: Friday, January 21, 2005 1:43 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] projection questions

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