[Mapserver-users] Help Request

[Ed McNierney]

Kieran -

MapServer is perfectly capable of handling data in different projections
and referenced to different datums.  I do it every day.

Do you have PROJ installed with datum shift support?

There's no such thing as "NAD27 Aware".  Each data set is expressed
relative to a particular datum, and it has no ability or need to know
about any other datum.  MapServer, using the PROJ library, can handle
all that.

If you "unproject" NAD27 data to NAD27 decimal degrees, and unproject
NAD83 data to decimal degrees, they WILL NOT line up.  That is expected
and correct behavior.

If the NAD27 data correctly line up with your streets (in either
projected or unprojected form) then you seem to have NAD27 street data.

There are two steps to this process.  Step 1 - Make sure you have
MapServer set up properly, with PROJ installed with datum-shift data
files, and Step 2 - Make sure you accurately represent the source
projection and datum of our data files.  If those two steps are done
correctly, everything will work.

I am a little concerned that you will end up whacking the data with a
hammer to make it fit, and end up with a self-consistent but incorrect
map display!

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: (978) 251-4242  Fax: (978) 251-1396
ed at topozone.com


-----Original Message-----
From: Kieran J. Ames [mailto:kames at keyspanenergy.com] 
Sent: Wednesday, March 05, 2003 5:25 PM
To: Ed McNierney; mapserver-users
Subject: Re: [Mapserver-users] Help Request


Ed,
Thanks for trying to help me today. I appreciate your time.
>From what I can gather, my street data (from ArcView Streetmaps) ARE
"NAD27 Aware". All research I have done thus far indicates that I can't
put these streets on the same map as these polygons and have them line
up.

I found that if I unproject the wetlands NAD27 data and wetlands NAD83
data of the same spatial area, (forget about including my street
information), they will not even line up with each other. However, the
unprojected NAD27 data DO line up with my streets. No wonder I couldn't
get the NAD83 polygons to line up!

I find it interesting, however, that mapserver doesn't know how to make
these layers play nicely together... (or put better, that I don't know
how to tell mapserver to make them play nicely together.)

I'm cc'ing the list with this so as to document the apparent
'dis-parity'
in some data sources. I'm sure there are several beginners (like myself)
that don't want to go through this time-intensive exercise.

Again... Thanks for your help.
Kieran



> > > Ed McNierney wrote:
> > > > Kieran -
> > > > Fire away - we work with DD street data on top of UTM data all
> the time.
> > > >         - Ed

"Kieran J. Ames" wrote:

> Would anyone be willing to contact me off-list to help resolve data
> alignment issues?
> Specifically, rendering decimal degree street data with UTM polygons.
> Thank you.
> kames at keyspanenergy.com
>
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