HPGN Datum?

[Dean C. Mikkelsen]

Hi Bill,

Here is some information for you about HPGN and HARN.

HPGN and HARN

HPGN (High Precision Geodetic Network) and HARN (High Accuracy Reference
Network) are synonymous. They are both names for the same project that
is focused on readjusting the NAD83 datum to a higher level of accuracy
state by state. HARN/HPGN is more accurate form of the North American
Datum.

What you are experiencing in Florida is that network of points that
defines the HPGN/HARN is based on NAD83, but that local states often try
to make denser grids and hence come up with better transformation
parameters.

The EPSG data model does incorporate the HARN information for Florida -
based also on the SPCS zones.

Looking at the files found in the PROJ.4 library, it appears as though
the code you are using epsg:26759 is based on NAD27. This is where I
would say the problem is occurring. If you use epsg:26959 this should
solve the problem and will match with the HARN/HPGN.

Cheers,
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 Bill Binko
Sent: April 24, 2005 8:25 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] HPGN Datum?

Hi everyone,

I found an excellent site for Florida map data:

http://www.fgdl.org/

Some of their data is a bit out of date, but there is a nice selection,
and it's all in the same projection, even from a wide variety of
sources.

My problem is that they use a datum that I haven't seen before "HPGN".
Here is the projection info from their metadata:

MAP PROJECTION PARAMETERS:

  Projection                           ALBERS
  Units                                METERS
  Datum                                HPGN
  Spheroid                             GRS1980
  1st standard parallel                24   0  0.000
  2nd standard parallel                31  30  0.000
  Central meridian                    -84   0  0.000
  Latitude of projection's origin      24   0  0.000
  False easting (meters)               400000.00000
  False northing (meters)              0.00000

After some looking around, and some advice from the board a few weeks
ago,
I used these projection instructions in Mapserver (CVS Head):

PROJECTION
      "+proj=aea"
      "+lat_1=24"
      "+lat_2=31.5"
      "+lat_0=24"
      "+lon_0=-84"
      "+x_0=400000"
      "+y_0=0"
      "+ellps=GRS80"
      "+datum=NAD83"
      "+units=m"
      "+no_defs"
END

It seemed the "NAD83" was close enough to the "HPGN" datam.  Now I'm
seeing small problem when combining any of the data from this source
with
others that are in the Florida West projection (espg:26759), and I'm
suspecting that the bad datum is the problem.

Actually, I'm hoping that's it because if it is, I'm confident someone
here can help me get it right: if not, it means the data's bad :-(

An example of the "wrongness" can be seen at:
http://www.binko.net/badProj.png

The roads should line up within the parcels.  The roads are in HPGN, and
the parcels are in espg:26759

Any help would as always be appreciated.


Bill

PS: Paul R: you did say you loved projection problems, right? :-b