[Gvsig_english] BUG: gvSIG 1.9 (Build 1253) Image Filtering

[Jorge Gaspar Sanz Salinas]

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On 17/12/09 23:24, Simon Cropper (Botanicus Australia Pty Ltd) wrote:
> Jorge Gaspar,
> 
> That is a good site. It explains CRS well, but leaves out the projection 
> element. It would be good if the SRS concept with projection + datum + 
> coordinates are described and illustrated in tandem.
> 

On this site is well explained and I'm pretty sure the explantaions on
the links provided on this thread are really better explanations than
mine.

Let me try my shot:

* An horizontal datum is the union of a geometric figure (an
ellipsoid) and some physic parameters like the total mass of the Earth
and others. This geometric figure can be design to fit the whole Earth
like WGS84 or ETRS89 or just an area like ED50 for Europe. The coming
of globlal navigation satellite systems made the global datums more
usual than the old local datums.

There's also a less known datum type for GIS people, vertical datums,
they are really important for i.e. civil engineerig.

* A coordinate system is the way you represent geometries on a
surface, there are dozens of coordinate systems and they have
associated a way (formulas) to convert from a geographic pair to a
projected pair of coordinates and the reverse. Examples: UTM,
Robinson, Lamert, etc...


You also have two types of operations with datums and coordinate systems:

* Conversion of coordinates: when you pass i.e. from lat/lon to UTM of
the SAME DATUM you are doing a conversion (the formulas I said). If
you want to convert from UTM to another coordinate system you usually
pass to lat/lon first and then back to the destiny coordinate system.

* Transformation: when you have to move from one datum to another
(i.e. ED50 to WGS84) you have to do a more complex operation that
involves to apply a mathematical algorithm (there are several like 7
parameters, a NTV grid, etc.) to pass from lat/lon in one datum to
lat/lon in another one. This graphic[1] from the crs-geo site explains
pretty well the chain of operations.

So, finally, the whole thing is that a "Coordinate Reference System
(CRS) is a coordinate system which is related to the Earth by a datum"
 (taken from an slide of a presentation[2] by one of the authors of
the jCRS gvSIG extension, at the first FOSS4G Girona meeting in 2006).

Besides that, there's a formal standard for this stuff: the ISO 19111
and a plethora of documents and old books on libraries and Internet to
look for :-)

Cheers!!
[1]
http://www.crs-geo.eu/SharedDocs/Bilder/CRS/schema-conv-trans-6crs,property=default.gif
[2] http://www.sigte.udg.edu/jornadassiglibre2007/comun/present/1.15.zip
- -- 
Jorge Gaspar Sanz Salinas
Ingeniero en Geodesia y Cartografía
http://www.prodevelop.es
tfno: +34 963 510 612

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