[Proj] Portuguese Modified Grid - Datum 73

[Jose Gonçalves]

Hello

Sorry, it was my mistake: it is in fact 39d40'. I edited the value in
the epsg file, to convert decimal degrees to DMS, and made the
mistake... The epsg file is ok.

Some history about that projection: that central point was chosen at a
mid latitude of the country and 1d00'00"E of the Lisbon Meridian
(9d07'54.862"W). The small false easting/northing of a few hundred
meters, which is quite unusual, was chosen in order to create
projected coordinates similar to projected coordinates in an older
datum (datum Lisboa).

Regards

José A. Gonçalves



2010/2/17 Roger Oberholtzer <roger at opq.se>:
> 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
> http://www.igeo.pt/produtos/Geodesia/inf_tecnica/sistemas_referencia/Datum_73.htm
>
> (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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