[gdal-dev] geographic vs projected coordinate system

[Phil Scadden]

You are correct in that imaging a round earth on a flat screen must 
involve projection. However, there is no one projection that is fit for 
all purposes so it is pretty normal to store data sets in GCS and 
project on-the-fly into whatever projection is fit for purpose. To 
complicate matters, describing the position of a point on the planet in 
lat/long means also defining a figure for the ellipsoid which is still 
an approximation for the real object, and furthermore our planet is 
dynamic so points move over time. This means there is no one, true GCS 
though for most purposes the WGS84 framework with a suitable time 
reference is new measurements. However historical data is likely to be 
measured on a local datum with earlier best-guess figure of earth but I 
digress.

There is nothing to stop you however from treating lat/long as 
rectangular coordinates and using that as a "projection". The projection 
figure is Plate Carree and this has no redeeming cartographic features 
as a projection. It is however, obviously exceedingly easy to calculate 
so  could be regarded as "programmer" or "computer resource" friendly. 
For a quick look at some data it okay, providing you understand what you 
are looking at.


Hi all, I am a novice to geospatialrelated terminology...and going 
through some literature to understand better.. Could someone please tell 
difference btwn geographic and projected coordinate system... I mean, I 
am not able to imagine how I can see an (satellite) image on screen ( 
flat surface) and say it is in geographic cordinate system (GCS)... as 
GCS means lat/long and I see it on a flat screen, so should it not be 
projected if I am seeing it on a flat screen? And if it is projected, 
then how can it be in GCS ? My doubt, perhaps , may sound very trivial 
for all the experts out there but I will really appreciate anything that 
will aid my understanding. Thanks. -- View this message in context: 
http://osgeo-org.1560.x6.nabble.com/geographic-vs-projected-coordinate-system-tp5059024.html 
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Phil Scadden, Senior Scientist GNS Science Ltd 764 Cumberland St, 
Private Bag 1930, Dunedin, New Zealand Ph +64 3 4799663, fax +64 3 477 5232

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