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Should We Downsample Aerial Imagery?

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#1
Jon B Ball

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Hi

I work as a carto/GIS technician for a municipality in Alberta, Canada, and am looking for some advice on image resolution versus intended usage.

We have just received a new lot of aerial imagery that covers our whole town (an area of about 8kmx8km). The imagery is high resolution - 9cm per pixel. We have various versions: a large mosaic image that covers the whole town and individual tiles (about 50) that cover a smaller area. The large image is about 100,000 by 100,000 pixels (e.g. 10 billion)! Large image is in MrSID format and the tiles are all available as ECW, MrSID and GeoTiff.

When printing maps, the largest map that we tend to print regularly is 84"x70", at a scale of about 1:3,750 if covering the whole town. We also print lots of maps at 42" width and at 36".

It seems to me that we have more pixels that we need when printing imagery of the whole town! (I tend to print at 288dpi an could print an image that is about 30 feet on each side if using 288 pixels per inch!)

My questions is this: What is the best practice when receiving high-res imagery such as this? Is it advisable to downsample versions of our imagery that are intended for particular print sizes and dpi's? If we downsample can this be done well in ArcGIS (ArcInfo license)? I am looking to end up with a practical image catalogue suitable for our various cartographic purposes.

We use ArcGIS(ArcInfo) v10.0(SP5) and I also have access to CS5 (but do not use much as yet).

Thank you in advance for any advice that you might be able to offer. I am only 7 months into my first carto job and greatly appreciate the chance to talk with a group that has the collective wisdom of this one.

Jon Ball

#2
elib

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Dear Jon;

I found online that you can do this resampling with ARCmap http://gis.stackexch...g-aerial-photos

I am not a GIS expert but have worked with remote sensing software that use similar methods to those indicated in the link. The software I have used is ENVI - If you do not have ENVI but have Matlab, you can can use it, especially if you have an image processing toolbox (Else, you can always program a code yourself in any programming language to do the resampling/downscaling).


Hope this helps

#3
AaronP

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There are several methods you can use in ArcGIS 10 to down sample your imagery:

  • Copy Raster will allow you to change both the pixel bit depth along with the cell size. Set the cell size in the environments section. You will have to confirm, but I believe the default resample method is nearest neighbor.
  • Resample allows you to choose the output cell size and the resampling method.

Both Copy Raster and Resample can be automated using Batch processing or through Python. If you have the means, I would advise using software such as Erdas Imagine that is optimized for working with remote sensing imagery. Best of luck.

#4
P.Raposo

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Hi Jon,

Yes, you can use Resample in ArcGIS to do this. Probably best to use cubic convolution as the necessary interpolation method; don't use nearest neighbor on imagery.

You could also just use the full-resolution imagery and let the inherent resolution difference at printing time handle itself, if you don't mind the large file size of the original imagery.

Cheers,
P




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