[TCMUG] Datum Transform Dakota Cnty => NAD83?

Mon May 5 17:15:02 PDT 2014

That looks similar to the Ramsey County projection.  In that case no 
datum shift (mean grid shift files) are needed (although NAD83 vs NAD83 
HARN+ might be a small difference).  What is different is using a custom 
ellipsoid instead of GRS80.  This is specified using the a and b 
parameters to proj4 which define the semi-major and semi-minor axes of 
the ellipsoid.   These are given in a different form in the Dakota 
County case as the semi-major axis and the flattening as 
6378421.989,298.2572133677353.

My notes from the Ramsey County case follow:

# MN Ramsey County Coordinates
# From Mike Murphy (City Surveyer) 6/6/2006
# From Mike Schadauer (MnDOT) 8/20/1996
# Also from "Map Projections and Parameters" from 
http://rocky.dot.state.mn.us (c)2002 MnDOT accessed 6/5/2006
# Conversion to from deg,min,sec to decimal degrees and Meters to U.S. 
Survey Feet by Jim Klassen 6/5/2006
#
# Ramsey County Coordinate Definition
#
# Projection Lambert Conformal Conic based on North American Datum 83 
with the 1986 correction
# Note: while MnDOT uses the HARN (NAD83 (1996)) corrections, Ramsey 
County does not. This amounts to
# about a 7/10th foot difference across the county (Mike M.) Ramsey Co. 
coordinates were designed to be
# accurate to 1:50000 (Mike M.) Also note the unit is the U.S. Survey 
Foot = 3937/1200 * meters.
#
# Parameters (verified matching from both Mike S. by way of Mike M. and 
from the above web site):
#
# STANDARD PARALELLS
#
# North Standard Parallel = 45 deg 08 min 00 sec. = 45.1333333333333333
# South Standard Parallel = 44 deg 53 min 00 sec. = 44.8333333333333333
#
# Longitude of Origin (Central Meridian) = -93 deg 23 min 00.0 sec. = 
-93.383333333333333
# Latitude Grid Origin = 44 deg 47 min 28 sec. = 44.791111111111
# False Northing = 30480.0610 Meters = 100000 U.S. survey feet
# False Easting = 152400.3048 Meters = 500000 U.S. survey feet
#
# To convert False Northing & Easting to U.S. survey feet multiply
#   Meters by 3.280833333
#
# Semi Major Axis = 6378418.941 Meters (Equatorial) = GRS80 
Semi-major-axis + ellipsoid_height = 6378137 + 281.941
# Semi Minor Axis = 6357033.310 Meters (Polar)
# Inverse Flatting 298.2572243
# Inverse Flatting = Semi Major Axis / (Semi Major Axis - Semi Minor Axis)
#  Semi Minor Axis = Semi Major Axis - (Semi Major Axis / Inverse Flatting)

<200068> +proj=lcc +lat_2=44.88333333333333 +lat_1=45.13333333333333 
+lat_0=44.79111111111111 +lon_0=-93.38333333333334 +x_0=152400.3048 
+y_0=30480.0610 +unit=sft +to_meter=0.30480060960122 +a=6378418.941 
+b=6357033.310 +no_defs <>

So I would guess the Dakota County line would be:
+proc=lcc +lat_2=44.91666666666666 +lat_1=44.51666666666667 
+lat_0=44.47194444444445 +lon_0=-93.31666666666666
+x_0=152400.3048 +y_0=30480.0610 +unit=sft +to_meter=0.30480060960122 
+a=6378421.989 +b=6357036.347

On 05/05/2014 04:18 PM, David Fawcett wrote:
> I am projecting some of the new awesomely open Dakota County spatial 
> data from the Dakota County custom SRS to EPSG 26915.
>
> It looks like a datum transform is in order, but I am having a hard 
> time coming up with one.  Any suggestions?
>
> Here is the .prj file contents for the SRS:
>
> PROJCS["MN_Dakota_Lambert_Conformal_Conic",GEOGCS["GCS_MN_Dakota",DATUM["D_MN_Dakota",SPHEROID["MN_Dakota",6378421.989,298.2572133677353]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",100000.0],PARAMETER["Central_Meridian",-93.31666666666666],PARAMETER["Standard_Parallel_1",44.51666666666667],PARAMETER["Standard_Parallel_2",44.91666666666666],PARAMETER["Latitude_Of_Origin",44.47194444444445],UNIT["Foot_US",0.3048006096012192]]
>
> For reference, here is the .prj for 26915:
>
> PROJCS["NAD_1983_UTM_Zone_15N",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-93],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["Meter",1]]
>
>
> It looks like they use the same spheroid.
>
> David.
>
>
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