Hi All,

I want to apply UTM projection to SRTM Raster tile, which has following extens:
Lat: 23-24 deg
Long: 73-74 deg

After selecting grid in GIS Tools,i select following options:
Category : Projected Coordinate System
Group : UTM WGS 1984
Name : WGS 1984 UTM Zone 23N

As per my uderstanding, this option is for assigning "ROW" {Latitude values} but there is no option for assigning "ZONE" {Longitude value}!!

and therefore, when i assign projection it corretly displays LAT values but wrong LONG values, it shows 14 deg values for longitude whereas my tile has LONG values varying in 73-74 degree!!

any suggestions please?

Hi Meetmukesh,

Lets first clarify what exactly you're trying to do.

So you have a raster image in SRTM format in lat/long (geographic coordinates) and you want to re-project the image into WGS 1984 UTM Zone 23N (projected coordinates) which I don't believe is the right projection for that lat/long, I'll explain why lower down.

The reason that you're not getting the results that you want is because you are using the "Assign Projection to Grid" tool. This tool does not re-project your raster's data it simply modifies the files specifications to indicate that the data is in a particular projection. This is useful when you work with data that is in an unknown projection, this happens some times when you download data from the web that do not come with projection specifications embedded in the file.

Assuming that your data already has a proper projection assigned to it (Which is no longer the case now that you've assigned it UTM Zone 23N) then what you want to do is "Reproject Grids" this will take the original raster data and transform it into a projected coordinates system.

Also you seem to be a bit confused about the UTM Zone system. UTM (Universal Transverse Mercator) is a type of projection (a Transverse Mercator projection), it can only cover a small band of the earth at a time from north pole to south pole. Because it can only cover a small portion of the earth UTM divides the surface of the earth into 60 vertical stripes called Zones (The number of the zone has no direct relation to the longitude of the zone). To make life easier the earth is also divided north and south. Take a look at this wikipedia page it explains it well [en.wikipedia.org]

Further more if you source data is at 23-24deg north and 73-74 deg east then it covers part of India and it is in UTM Zone 43N

Hope that helps clear things up.

Post back here if you have any other questions about this or if this solved your problem.

Brian



Edited 1 time(s). Last edit at 03/26/2010 09:37AM by bmarch.

Two remarks to add to Brian's excellent description:

[www.dmap.co.uk] is a good map of UTM zones

There is a direct relation between longitude and UTM zone:

zone = ceiling((longitude + 180)/6)
where longitudes are in the range -179 to +180, west longitudes treated as negative, east as positive.

Hi Chris,

Your right, there is a direct relationship, its just not a simple 1 to 1. Also that little bit of code is a great way of describing it.

Brian

yeah , thank you so much Bmarch.
i did it successfully.



Edited 1 time(s). Last edit at 03/30/2010 08:09AM by meetmukesh.