Hi List,

Thanks to all those who replied. It is apparent that there are several different
ways to obtain the average (mean) slope of an area of grid cells. I've ended up
going with a combination of Grid Statistics and Summarise Zones (see details
below). I must make mention of the advice provided by Bill Hubber regarding cell
size and scale issues, to Michael Kohler for his thoughts and extension, and
finally Michael S. Batcher for his ideas on small areas of extreme values.

This is a summarised summary (if that makes sense!) as the whole document is
quite long.So if you would like a more detailed copy then please email me at
Nick_Bauer@enterprise.canberra.edu.au

1. GRID STATISTICS
aGrid.getStatistics which will give you a list of stats including the mean. The
2 index in the list is the mean value - which you can get using alist.get(2)

Ravi Narayanan and Jeff Jenness

2. SUMMARISE ZONES
I just finished a similar hydro modeling project and found this to be a very
simple process with no Avenue programming required.  I assume you have a polygon
file with your watersheds (with a unique id column) and a slope grid.  With SA
loaded, under the Analysis menu pick Summarize Zones.  Follow the menus to pick
your polygon watershed theme and a grid theme to summarize based on the slope
grid.  The result is a table with several statistical columns including one that
is the mean.  Join this table to your watershed theme, create a new column in
the watershed table named mean_slope and use the calculator to populate this
column with the joined mean column.  Now you can remove the table join and you
will have the mean_slope in your watershed table.

Mike Schaefer

3. AREA WEIGHTED AVERAGES
Donald C.E. Robinson

4. SOME THOUGHTS ON CELL SIZE AND SCALE ISSUES
Bill Huber

5. ARCFLOW EXTENSION AND ZONAL STATS
Michael Kohler (see his ARCFLOW ext at www.spatial-online.com)

6. SOME THOUGHTS ON SMALL AREAS OF EXTREME SLOPE VALUES
Michael S. Batcher

7. ZONALSTATS IN AVENUE
Andrew Loughhead, Chris Fabian and Chip Hankley

Thanks again
Nick Bauer
CRC Freshwater Ecology
University of Canberra
Nick_Bauer@enterprise.canberra.edu.au