projection questions

[Stephen Woodbridge]

Jeff Portwine wrote:
> Well, I guess I didn't mean specifically "New England" but the general
> area.... my map includes most of Pennsylvania, New Jersey, Massachusetts,
> Conneticut, Vermont, Rhode Island, New York, and part of Maine.
>
> It is entirely possible that as you said i'm being misled by the Map Point
> map... and that I just have to get used to how this looks , but
> regardless I
> need to understand what's going on so that when the people I'm doing this
> for ask me I can explain the difference :)
>
> I put up a quick and dirty web page:   http://www.aiedail.net/maps  so that
> you can see exactly what I'm talking about (I didn't want to attach images
> to an email for the entire list).    I hope that these images will
> demonstrate what i've been trying to write in text.

It looks like your map is in a latlong projection and the mappoint map
(just guessing because I have a large printed wall map) may be in a
transverse Mercator projection.

> I'm not actually using mappoint data to create the map, so I have no idea
> how they project the data when they generate maps...  but I can't really
> seem to make my output look any different regardless of what I use for
> output projection.

You can change you output projection all you want it it will not change
the look of the map one bit unless you set the input projection of each
layer. If you data is all in lat,long then you should set the PROJECTION
block in each layer to PROJECTION "proj=latlong" END and then add a
PROJECTION ... END block to the map section of the mapfile for the
output projection.

> If there is no way to make it look the same that's probably OK, i'm just
> trying to understand what's happening.
>
> Thanks for the time, hope  the picture posting helps.
>
> -Jeff
>
>
>> ----- Original Message -----
>> From: "Ed McNierney" <ed at TOPOZONE.COM>
>> To: <MAPSERVER-USERS at LISTS.UMN.EDU>
>> Sent: Monday, January 24, 2005 2:33 PM
>> Subject: Re: [UMN_MAPSERVER-USERS] projection questions
>>
>>
>> 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
>>
>>
>