So far all the GBR maps I found contained unprojected features and only used 'WGS84' or 'Geocentric Datum of Australia 1994' datum-based references. This is not really a problem for display but it becomes a problem for distances and areas calculations.
thanks in advance!
The question is really interesting.
There's no projection system which preserve both of these characteristics. Some projections preserve a constant ratio between the area on the Earth and the area on the projection on your map (said to be equal-area). Other projections are said to be conformal and preserve the angle. They also preserve the distances around a point. Lastly you have some projections which make a compromise between both.
Depending on your applications, you need to work with different projection systems. Also, it depends on the precision needed (exact calculation or just an approximation?).
Salut Quentin
Merci pour ta reponse! Sympa aussi de voir qu'un liegeois soit le premier a me repondre! :-) Ma question etait un peu vague en effet. Merci de me rappeler la theorie sur les differentes projections (cours de geometronique de Debouche a ULg/Gembloux) pour preserver, dans ce cas-ci, les surfaces plutot que les distances our les formes. Cependant ma question etait plus pratique: je cherche simplement a connaitre le systeme de projection le plus adapte pour cette zone-la. Vu la taille du GBR (>2300km de long et surface de >350000 km2) ce n'est pas facile de trouver une projection qui limite les distorsions. Certaines existent soit pour toute l'Australie, soit pour des sections du GBR uniquement...
Je pensais peut-etre utiliser la projection suivante: "GDA 1994 MGA Zone 55" (http://georepository.com/crs_28355/GDA94-MGA-zone-55.html). C'est une projection "Transverse Mercator". La zone 55 recouvre presque tout le GBR donc a moins de trouver quelque chose de mieux j'imagine que ca pourrait faire l'affaire, non? Qu'en penses-tu?
Je garde en tout cas ta remarque tres judicieuse! A+
Bonjour, ok je comprends. Désolé je ne vais pas te répondre parce que je ne connais pas vraiment la région et je ne souhaite pas être responsable d'une erreur. Le mieux pour ce genre de questionnement reste de calculer les indices de déformation (cercle de Tissot, modules linaires, altérations de surfaces, ...) de la projection que tu proposes et voir si ça convient avec ton cahier des charges.
Bien à toi.
Q.Glaude
my dear sir
theoretically i studied that two standard parallel conical projection or one standard parallel conical projection system will be give you closest accuracy in shape and distance calculation in such areas.
Dear Jeremy,
Note that to calculate areas using GIS, you will need to use a projected coordinate system. The Universal Transverse Mercator (UTM) projection is one of the best-compromise projections based on definite zones, so you must know the UTM zone that represents the region you are mapping. UTM uses a projected metric system and will enable you to calculate areas easily.
Putting all your remarks together while searching for the right projection on http://georepository.com/home.html, I shortlisted 3 systems that seem most relevant:
#1 & #2 are Australia-wide whereas #3 stretches across the longitudes I am mostly interested in (141E to 152E). #1 & #2 preserve the areas, which I am looking for since I want to calculate areas of different ecosystems in that region.
I guess I'll just have to try and compare results for these 3 options now!
Thanks everyone for all your very useful comments!
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