RE: circular smoothing and relative difference

[Stuart Nixon]

Ken,   I saw you email a week ago and only now have had a chance to respond to your need to min/max averages over variable filter areas. Several thoughts:   -    Have a look at the various DEM filters, algorithms from Jo Wood, that John Dehls kindly     coded and provided as user based C filters specifically for DEM work (thanks John!).     These do some interesting work for DEM enhancements.   -    As it look like you have already found, C based filter (kernel) functions will be the best     approach to your specific requirement.   -    Have a look at <ER Mapper path>\usercode\kernel\c for some examples,     and the "Writing compiling usercode on PCs" section of the "Customizing" manual   -    When you define the kernel interface to your C function, you specify the default filter     size.  However, you can override this by clicking on the filter icon in the algorithm window,     and changing the size of the filter at a later time (assuming your C function can handle     different kernel sizes, of course).  Various examples show how your code gets passed     the array of numbers, including the size.  "Average.c" is a good simple example.   -    In your case, you probably want to create an circular average (perhaps guassian) matrix to     multiply the filter input values against.   -    If you have some init work (like the point above) that only needs to be done once,     you can add a "<filter>_init(int rows, int cols)" function. Lets say your function is     called "avgminmax()", then your init function would be called "avgminmax_init()".     Ditto for a "<filter>_finish()" function if you need it.   -    The manual gives some examples of things like digging out cell size if you need that     for some reason.   -    You might also want to look at the "Weighted local mean adjustment kernel" which     comes close to doing what you want (source is in wlocalmadj.c).  Created by Dr. Liu     (his work is always worth exploring).     Don't let all the above scare you off - it is actually really easy to link user code filters and formulae extensions to ER Mapper, and the example files should give you most of what you need.  If not, there are a number of power-user ER Mapper people who are on the list who would be happy to help (as I and other ERM people will of course as well).     Kind regards,   Stuart     G'day all   Does anyone know how I can make a filter that averages pixel values over a circular area with a user defined radius. I also need a filter that does a (max-min) calculation in the same circle. I am tyring to compare mean elevation versus local relief over a DEM.   Cheers Ken Hickey ------------------------------------------------------------------ Dr Kenneth Hickey Mineral Deposit Research Unit (MDRU) Department of Earth and Ocean Sciences University of British Columbia 6339 Stores Rd, Vancouver, BC  V6T 1Z4, Canada phone +1-604-822-3765; Fax +1-604-822-6088 email: [EMAIL PROTECTED]   .