Author Topic: How to define an ISIS 2.1 cub so it will overlay in a GIS  (Read 7950 times)

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How to define an ISIS 2.1 cub so it will overlay in a GIS
« on: March 02, 2005, 09:12:13 PM »
I would like to prep an image in ISIS so that I can bring it into a GIS. How far do I need to process an image before it will register?

thare

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How to define an ISIS 2.1 cub so it will overlay in a GIS
« Reply #1 on: March 02, 2005, 09:18:29 PM »
The most important requirement for an ISIS image is that it is a level 2 image and that it has a projection set in the labels. This allows one to calculate the image registration values (usually in projected meters) for GIS and other packages. There are also PERL scripts that one can run on the ISIS files to extract this information into header and world files. These programs now come with ISIS.

Datasets that are not projected in ISIS:

Datasets not generated in ISIS that are already geometrically corrected need to be converted to an ISIS format and then have the ISIS program 'lev2lab' run on the file. This will generate ISIS projection labels for the file. This program should be run instead of the older 'maplab'. 'Maplab' does not support the ocentric latitude system - this is very important for Mars.

Parameters that need to be set for lev2lab are:
"from", "targdef", "mappars", "lonsys", "latsys", "lon,lat,sample,line"

targdef: Defines which planet.

mappars: defines the projection and parameters of that projection. In TAE type 'help mappars' for more.

lonsys: defines the longitude system - either 180 or 360. To match different datasets this must be same across all data. If the data is Simple Cylindrical (Equirectangular) 0 to 360 (lonsys = 360), most programs will need to have a center longitude of 180. If the data is Simple Cylindrical (Equirectangular) -180 to 180 (lonsys = 180), most programs will need to have a center longitude of 0.

latsys: either ocentric or ographic. Again this must be same across all data. Most commercial packages do not directly support ocentric. This doesn't mean you can't use ocentric values. But just understand that there could be registration problems if you add an automatic grid or you try to project it. Again, any other data that you plan to match to, needs to be ocentric.

lat,lon,line,sample keywords define a single known control point that tells ISIS for one pixel what is its' lat,lon value.

Global Simple Cylindrical file, lonsys=180:
mappars="simp:0,ocentric"
lonsys=180
latsys=ocentric
lat=90
lon=-180
line=0.5
sample=0.5

Global Simple Cylindrical file, lonsys=360:
targdef=mars
mappars="simp:180,ocentric"
lonsys=360
latsys=ocentric
lat=90
lon=0
line=0.5
sample=0.5

Mercator lat=(0,30),lon=(0,40):
targdef=europa
mappars="merc:0,0,ographic"
lonsys=180
latsys=ographic
lat=30
lon=0
line=0.5
sample=0.5

polar (clat=90, clon=0 number pixels 1000x1000):
targdef=mars
mappars="pola:90,0,ocentric"
lonsys=360
latsys=ocentric
lat=90
lon=0
line=500.5
sample=500.5

Once the "GROUP = IMAGE_MAP_PROJECTION" is set in the labels, the data has enough information to be georeferenced in a GIS.