HPGN Datum? (fwd)

[Dean C. Mikkelsen]

Hi Frank and Bill,

Frank you are on the right track I do believe. I will gladly take a look
at it as well if you want someone to hit ideas back and forth with.

HPGN and NAD83 are virtually equivalent for mapping purposes.

Allow me to ask a dumb question. I'm pretty new to MapServer (just
started playing with it this month - but have been doing geodesy
consulting for 10+ years), but should the Layer (Roads) not have the
values of NAD27, as that is what we want it converted to? Hence the map
shows up in the same datum. The assumption here is that MapServer
Mapscript reads the .prj file, which is defined in HPGN, but we want the
data converted to NAD27 for this layer. Or does having the projection
defined at the top take care of this and we have to define what the
layer's datum is?

Sorry if this sounded confusing, but it looks like a datum definition
problem and the solution is finding out where it should be defined for a
mixed datum dataset (two files w/different datum's) and whether the
CONUS files (as Frank mentioned) are being read to do the conversion.

Kind regards,
Dean


Dean C. Mikkelsen,B.Sc.,P.Eng.
Principal Consultant
Terra ETL Ltd. - Consultants on GIS & Geodesy
Vancouver, B.C., Canada
Phone: 604-929-8630
E-mail: dcmikkelsen at shaw.ca



-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Frank Warmerdam
Sent: April 25, 2005 8:51 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] HPGN Datum? (fwd)

On 4/25/05, Bill Binko <bill at binko.net> wrote:
> Ok, well, I'm putting a stripped down version of my mapfile below.  It
has
> only two layers: one in each of the two projections.  I tried
replacing
> the "init=espg:27659" with the actual contents of my
/usr/share/proj/espg
> in both the MAP level and the LAYER level, but it came back very
messed
> up!  So I thought that perhaps I'm going at this wrong: I created
these
> mapfiles before I had any clue and haven't come back to them in a
while.
> Would you guys mind checking my setup on these?

Bill,

Your declarations look fine from what I know of your problem (I hadn't
been following the discussion closely).   But my guess is that
the Roads layer is just not being converted back to NAD27 properly.

Do you have the file /usr/local/share/proj/conus on your system?

One way of testing would be to change the roads declaration to be
NAD27 and see if that makes any difference.  If there is no noticable
effect, then it likely means that the datum shift file is not being
found
and applied.
                         PROJECTION
                               "+proj=aea"
                               "+lat_1=24"
                               "+lat_2=31.5"
                               "+lat_0=24"
                               "+lon_0=-84"
                               "+x_0=400000"
                               "+y_0=0"
                               "+datum=NAD27"
                               "+units=m"
                               "+no_defs"
                       END

> I'd send the data, but it's 49MB, and I'm not sure how to extract a
subset
> without messing it up and/or removing the problem.

If you still can't get things working, bundle stuff up and let me know
where to grab it and I'll take a crack at the problem.

Best regards,
--
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------
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent