[GRASS-dev] r.watershed question

Michael Barton michael.barton at asu.edu
Sat Dec 20 02:15:21 EST 2008


Should r.watershed be giving negative accumulation values? I ran it on  
the spearfish 30m DEM and got results that are increasingly positive  
downstream for  one drainage (as I would expect) and increasingly  
negative downstream for other drainages (I would not expect this). Do  
I misunderstand what accumulation is supposed to do?

Here is r.info for the results.

r.info atest_rwaccum
   
+ 
----------------------------------------------------------------------------+
  | Layer:    atest_rwaccum                  Date: Sat Dec 20 00:08:40  
2008    |
  | Mapset:   PERMANENT                      Login of Creator:  
cmbarton        |
  | Location:  
Spearfish60_test                                                 |
  | DataBase: /Users/Shared/ 
grassdata                                          |
  | Title:    Watershed accumulation: overland flow that traverses  
each cell ( |
  | Timestamp:  
none                                                            |
   
| 
----------------------------------------------------------------------------|
   
| 
                                                                             |
  |   Type of Map:  raster               Number of Categories:  
81709           |
  |   Data Type:     
CELL                                                       |
  |   Rows:          
466                                                        |
  |   Columns:       
633                                                        |
  |   Total Cells:   
294978                                                     |
  |        Projection: UTM (zone  
13)                                           |
  |            N:    4928000    S:    4914020   Res:     
30                     |
  |            E:     609000    W:     590010   Res:     
30                     |
  |   Range of data:    min = -90540  max =  
81709                              |
   
| 
                                                                             |
  |   Data  
Source:                                                             |
  |     
elevation 
.dem at PERMANENT                                                 |
   
| 
                                                                             |
   
| 
                                                                             |
  |   Data  
Description:                                                        |
  |    generated by  
r.watershed                                                |
   
| 
                                                                             |
  |    
Comments 
:                                                                |
  |    Processing mode: All in  
RAM                                             |
   
| 
                                                                             |
  |    r.watershed elevation="elevation.dem at PERMANENT"  
accumulation="atest_\   |
  |    rwaccum"  
memory=300                                                     |
   
| 
                                                                             |
   
+ 
----------------------------------------------------------------------------+
(Sat Dec 20 00:09:19 2008) Command finished (0 sec)

I am running this on my Mac using GRASS 6.4SVN from 10 December. If  
there was a major bug fix since that date, I can recompile tomorrow.

Michael



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