[Proj] Belge 1972 / Belgian Lambert 72 (31370) -towgs84parameters

[Jan Hartmann]

Nice. Our Belgian friends seem to have changed their national 
projection. I don't understand this page (Dutch: 
http://www.ngi.be/FR/FR4-4.shtm; French: 
http://www.ngi.be/FR/FR2-1-7.shtm, no English need apply). It says:

"Due to the too small deviation (1 kilometer) between the coordinates of 
the Lambert 1972 projection and those of the Lambert 2005 projection, 
the NGI has changed the parameters of the Lambert 2005 projection, and 
created the Lambert 2008 projection".

What they did was just adding 499.000 to both x and y coordinates of 
Lambert 2005.

Lambert 1972 is the projection as defined in PROJ's epsg file (nr 
31370), derived from the EPSG database (coordinate reference nr 411). 
Its datum is intl (=Hayford). First standard parallel is 51.166667, 
second standard parallel 49.833333, central parallel 90 (?), central 
meridian 4.367486666. x_0 = 150000.01256, y_0 = 5400088.4378

Lambert 2005, as defined on the above page (nothing in the EPSG 
database): datum GRS80 (=WGS84, approximately). First standard parallel 
51.166667, second standard parallel 49.833333, central parallel 
50.7977777, central meridian 4.35916666666. x_0 = 150328, y_0 = 166262

Lambert 2008 is Lambert 2005 with 499 km added to both x and y coordinates

So, without additional information, I really don't see the relationship 
between the 1972, 2005 and 2008 systems. What's the difference between 
1972 and 2005, and why did they add 500 km in 2008? Did they change each 
and every coordinate on each and every Belgian map in 2005 and are they 
changing it again in 2008? Has anyone ever seen a Lambert 2005 
coordinate? They are not in the EPSG database.

If anyone can make sense of this gobbledygook, I would be happy to know.

Jan




On 27-1-2010 20:12, OvV_HN wrote:
>
> ----- Original Message ----- From: "Jan Hartmann" <j.l.h.hartmann at uva.nl>
> To: "PROJ.4 and general Projections Discussions" 
> <proj at lists.maptools.org>
> Cc: "OvV_HN" <ovv at hetnet.nl>
> Sent: Wednesday, January 27, 2010 7:59 PM
> Subject: Re: [Proj] Belge 1972 / Belgian Lambert 72 (31370) 
> -towgs84parameters
>
>
>> Thanks Oscar, good to know. How did you find this page? Is it 
>> reachable from within the ngi site or did you find it with Google.
>>
>> Jan
>>
>
>
> The path starting from the Dutch language home page is:
> home > know-how > geodetische diensten > Lambert 2008
> http://www.ngi.be/NL/NL2-1-7.shtm
>
> Scroll down to:
> "Relatie tussen de geodetische referentiesystemen van 1972 en 2008"
> ...
> "> uitgebeider schema in pdf"
>
> This last link goes to the page I mentioned.
>
>
> Oscar van Vlijmen
>
>