1

I have knowledge enough to play around Proj4s string but I have not found any question about this yet.

I have these two Proj4 strings.

+proj=eqdc +lat_0=39 +lon_0=-96 +lat_1=33 +lat_2=45 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=-0.9956,1.9013,0.5215,0.025915,0.009416,0.0011599,-0.00062 +units=m +no_defs

and this one below (+datum=NAD83 is added in front of +ellps=... +towgs84=...)

+proj=eqdc +lat_0=39 +lon_0=-96 +lat_1=33 +lat_2=45 +x_0=0 +y_0=0 +datum=NAD83 +ellps=GRS80 +towgs84=-0.9956,1.9013,0.5215,0.025915,0.009416,0.0011599,-0.00062 +units=m +no_deft

In case you wonder, The 7-parameters argument in both strings is from this post.

I wonder if:

  1. In the 2nd string, will +ellps=... +towgs83=... override +datum=NAD83 during transforming projection?

  2. I have two file about the same data projected by 1st string and 2nd string. I am going to transform them to any NAD83-based projection or NAD83 latlong(EPSG-4269). Will Proj4 transform the data projected by 2nd string to the new NAD83-based projection with or without datum shift because I had +datum=NAD83 in the Proj4 string and in front of +ellps=... +towgs84=...?

2.1 After projecting 1st-string-projected data and 2nd-string-projected data to the same NAD83 projection, will or will not I have two same results?

1

I can't say how it goes in theory but it is easy to test what happens in practice with gdaltransform utility http://www.gdal.org/gdaltransform.html

gdaltransform -s_srs EPSG:4326 -t_srs "+proj=eqdc +lat_0=39 +lon_0=-96 +lat_1=33 +lat_2=45 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=-0.9956,1.9013,0.5215,0.025915,0.009416,0.0011599,-0.00062 +units=m +no_defs"
-80 36
Result:
1429512.75999231 -207252.541637988 1.35481430124491

gdaltransform -s_srs EPSG:4326 -t_srs "+proj=eqdc +lat_0=39 +lon_0=-96 +lat_1=33 +lat_2=45 +x_0=0 +y_0=0 +datum=NAD83 +ellps=GRS80 +towgs84=-0.9956,1.9013,0.5215,0.025915,0.009416,0.0011599,-0.00062 +units=m +no_deft"
-80 36
Result:
1429512.75999231 -207252.541637988 1.35481430124491

Result is the same to the very last decimal.

1
  • I guess +ellips=GRS80 +towgs84=... in the latter place overrides the previous +datum=NAD83 Nov 6 '17 at 14:40

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