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Clipping Landsat Imagery?

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#1
dWoolley

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Hello all.
Please forgive what I suspect is an incredibly basic question- my remote sensing and image processing skills are entirely self taught thus far- that being said-

I'm working with Landsat TM data and attempting to use ESRI shapefiles to clip a study area from the Landsat TIF images in ArcMap. My shapefiles did not originally have a coordinate system defined, so I defined them used the coordinate system from the TIF (WGS 1984 UTM 16N) but so far am not having any luck in getting anything resembling a coherent match. I can't even get the various shapefiles and TIF to project at the same scale, let alone make any useful clipping from them.

As I say, I suspect I'm missing something fundamental, perhaps several things, but any thoughts or resources on techniques to accomplish this task (using shapefiles to clip satellite imagery) would be helpful.

Thanks and best to all.
David

#2
alanb

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David,
I am certainly no expert, but I can suggest how I might start tackling the problem.
The first problem is your shapefile that you want to use to clip your rasters. They didn't have a projection defined, so you defined one based on the projection of the imagery. The problem is this wont Prject the data, just tell the data how to display its x and y coordinates. I would sugest you preview your shapefile in ArcCatelogue and run your cursor over preview looking at the coordinates change in the bottom right hand corner of the screen. if you are seeing something like 35.3535 -83.2345 and as you move the cursor the decimals go up and down then you can be fairly sure that the original projection (which it seems to have lost) was 'Geographic'. If that was the case try redefining the shapefile as WGS84 or a datum that you use in your part of the world. If that was the case then when you defined the projection to UTM, the software read Degrees and Seconds in Meters of a UTM zone (16N) and therefore would always put your shape in the very bottom left corner of the of the Zone. If this then brings the Shapefile to very close but not quite spot on, try another couple of common Geographic Projection datums. until you are satisfied. Then reproject into UTM 16N.

As for clipping the raster, you are right it is a good Idea to have your shape and raster in the same projection. With both loaded into ArcMap open your toolboxes, click on Data Management Tools, click on Raster, click on Raster processing, and select the clip tool. For Input Raster navigate to your image, for Output Extent select your clip shapefile. Don't forget to check the "Use input Features for Clipping Geometry (optional) box. This will clip the raster to the shape of your shapefile. Unchecked this box will make a clip based on the minimum and maximum horizontal and vertical extent of the file (therefore a rectangle).

Hope this helps, with my limited knowledge others may have a slicker approach to solving your projection problems.
Alan

#3
alanb

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David,
I have just checked the ESRI shapefiles that come with Arc. They are indeed projected to Geographic, GCS_WGS_1984. Somehow they have lost the projection info. So 'define project' as GCS_WGS_1984, then project the file to UTM16N. The simplest way to understand this is that 'defining the projection' just tells the software how to read the xy coordinates, where as actually 'projecting' the file changes the coordinates themselves and changes the geometry of the file.
Cheers, Alan

#4
BioGeoMan

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Another option for clipping rasters (assuming you have Spatial Analyst extension) is to Clip by Mask located in the Toolbox>Spatial Analyst Tools>Extraction>Extract by Mask.

Good luck!

Michael Scisco

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505-603-3636
biogeocreations.com


#5
alanb

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Thats right, and yet another method is to set up the spatial analysts options (especially analysis mask) and use raster calculator to recalculate the raster only within the area bound by a polygon of your designation. I describe this method here

http://mappingcenter...a...swers&q=584

Apparently this is a good method of maintaining the exact resolution and cell possitioning of the original data. It is quite possible, however, that the tool that BioGeoMan describes above is simply the script for doing exactly the same thing, my method just being the long way round.
Cheers,
Alan

#6
dWoolley

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Thats right, and yet another method is to set up the spatial analysts options (especially analysis mask) and use raster calculator to recalculate the raster only within the area bound by a polygon of your designation. I describe this method here

http://mappingcenter...a...swers&q=584

Apparently this is a good method of maintaining the exact resolution and cell possitioning of the original data. It is quite possible, however, that the tool that BioGeoMan describes above is simply the script for doing exactly the same thing, my method just being the long way round.
Cheers,
Alan



Thanks very much for the replies and the information. You've given me some ideas to try and I think you're on to something with the problem you mentioned in your first reply, as the .TIF and the .SHP are far, far away from each other and in roughly the places you predicted. I'm about to sit down and give it another go using some of your ideas. I've also found another source of shapefiles for my study area that might be a bit more useful than the first batch!

Best regards,
David

#7
dWoolley

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Another option for clipping rasters (assuming you have Spatial Analyst extension) is to Clip by Mask located in the Toolbox>Spatial Analyst Tools>Extraction>Extract by Mask.

Good luck!


Thanks for the suggestion. I've used the 'extract by mask' tool quite a bit and have done a sort of workaround here by creating an empty shapefile in ArcCatalog and using that as an approximation of my study area to extract from the original .TIF. Not ideal but it gets the ball rolling. Thanks again!

David




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