[Proj] Proj4 datum transform assumption Question

Joel_Cusick at nps.gov Joel_Cusick at nps.gov
Mon Sep 22 19:49:15 EDT 2008

Brand new subscriber, but long time user of DNRGarmin.  I am trying to
resolve the datum transform signage in Proj 4 projection engine that is
being used by Minnesota's DNRGarmin software.  One can open up the ESRI or
EPSG files and directly observe the 7 parameters for datums.  Since
DNRGarmin loads to and from Garmins using Garmin proprietary protocols, the
Proj4 text files are used to transform to or from WGS84 depending on data
flow.  Loading from a Garmin, its expected to transform from WGS84 to
NAD83.  When Sending to a Garmin from a NAD83 shape, its expected to
transform from NAD83 to WGS84.

question:  Does Proj4 assume data is going to WGS84?  Using the same _5
ESRI transform parameters, but written in DNRGarmin, the syntax used is
"+towgs84".  This infers that Proj4 is written as a default From NAD83 to
WGS84.  This is counterintuitive to documentation presented by Soler/Snay
article (see Table 1, Page 51, last column
http://www.ngs.noaa.gov/CORS/Articles/SolerSnayASCE.pdf ) for the transform
parameters from ITR96 to NAD83.  When comparing the articles's 7
parameters, they are exactly opposite in signage to the DNRGarmin program.
Just want to confirm how exactly Proj4 is coded.


>From DNRGarmin ESRI nad file

# NAD_1983_To_WGS_1984_5
<1515> +proj=longlat +ellps=GRS80
+towgs84=-0.991,1.9072,0.5129,-0.02579,-0.00965,-0.01166,0 +no_defs
no_defs <>
Joel Cusick
National Park Service
GIS Specialist/Alaska Regional Office
240 West. 5th Avenue, Anchorage, AK - 99501
(907) 644-3549  joel_cusick at nps.gov
+61 13' 01.28", -149 53' 10.58";  (NAD83 CORS96)

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