When you talk of the ArcGIS FocalStatistics tool, to which statistics do you
refer?
ArcGIS Spatial Analyst is built on C Dana Tomlin's Map Algebra and the ArcGIS
Workstation command line implementation 'Grid Algebra'. In Map Algebra, Focal
functions operate on an immediate neighbourhood of specified size, yielding a
result which is stored in the output 'focal' or central cell. FocalStatistics
provides a single tool to deliver the functions FocalMean, FocalMedian,
FocalMinimum, FocalMaximum, FocalMinority, FocalMajority and FocalRange.
You talk of 'blurring' the raster, however each of these functions has a
specific purpose related to its result - FocalMean and FocalMedian, for example,
deliver a smoothing operation; FocalRange delivers what is, in effect, a measure
of the smoothness of the neighbourhood; and so on.
If you are writing a raster model, for example, using GDAL to manage the data,
then you can implement any of these when using GDAL, but you will not find GDAL
functions ready for you: GDAL provides you with the essential tools by which to
access and manipulate raster data - read and write, transformation, etc., but it
is up to you what you do at cell level. GDAL is not a GIS, however, and
therefore does not provide the same range of tools as ArcGIS - which, for
example, incorporates and uses GDAL internally for some raster data access.
If you need to implement such a model, then I would urge you to read Tomlin's
papers first, so that you understand what it is that you have been using in
ArcGIS. Start with the book 'Geographic Information Systems and Cartographic
Modelling', C Dana Tomlin, 1990, Eaglesfield Cliffs NJ: Prentice Hall.
Best wishes,
Peter
m k wrote:
Much of my GIS vocabulary comes from ArcGIS. ArcGIS has a Spatial Analyst Tool called
Focal Statistics. It basically "blurs" the raster by redefining cells based on
a search area (either circle or square), and the values that occur in these surroundings
(you can also control how big the area is with a parameter). I've found this process
useful to prepare a raster for hillshading.
I am wondering if there is an equivalent tool in the GDAL library?
Thank you
-m
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Peter J Halls, GIS Advisor, University of York
Telephone: 01904 433806 Fax: 01904 433740
Snail mail: IT Services, University of York, Heslington, York YO10 5DD
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