[Proj] Portuguese Modified Grid - Datum 73

Roger Oberholtzer roger at opq.se
Wed Feb 17 04:27:16 EST 2010

On Tue, 2010-02-16 at 17:34 +0000, Jose Gonçalves wrote:
> Hello
> The proj string you refer is not correct. What is called in the EPSG
> database "Portuguese Modified Grid - Datum 73" (epsg code 27492) is
> the projection used in Portugal for the local datum called Datum 73.
> Datum 73 was established in 1973 and is based on the Hayford
> ellipsoid.
> The correct definition of the projection can be found in the epsg file
> distributed with proj:

I am confused about the lat_0 term. All references for this say that
this is 39º 40’ 00’’ N. Also, in the files I have with proj, it is
defined as:

<27492> +proj=tmerc +lat_0=39.66666666666666 +lon_0=-8.131906111111112
+k=1.000000 +x_0=180.598 +y_0=-86.98999999999999 +ellps=intl +units=m
+no_defs  no_defs <>

Is it standard to use 39º 00’ 00’’ N instead? The people I am doing this
for have referenced

(Note: the reason I missed this entry in proj at the start and asked the
original question was that I looked for "Portugal" in the file. I never
thought to look for "Portuguese".)

> # Datum 73 / Modified Portuguese Grid (deprecated)
> <27492> +proj=tmerc +lat_0=39d00' +lon_0=-8.131906111111112 +k=1
> +x_0=180.598 +y_0=-86.98999999999999 +ellps=intl +units=m +no_defs  <>
> It is a Transverse Mercator projection with central point:
>     lat_0=39d00'00.000"N
>     lon_0= 8d07'54.862"W
> No scale factor in central meridian is applied (+k_0=1)
> It has a small false easting/northing
>     x_0=180.598
>     y_0=-86.990

Roger Oberholtzer

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