[GRASSLIST:924] interpolation & shading problem/question

[Brent Wood]

I'm slowly getting to grips with the idiosyncrasies of GRASS, having used
other GIS & mapping tools for several years.

My current problem lies in getting a good looking hillshade like the
spearfish dataset provides.


Hopefully I've included all the relevannt info below....



I have imported an xyz dataset into GRASS with:
cat mandes_.LI | awk '{print $2, $3, $4, $4}' | s.in.ascii andes d=3

The file mandes_.LI comprises N X Y Z values, where N is a record ID that
I'm omitting when I build the site file. The Z value is depth in
meters, X & Y are long/latitude in decimal degrees.


The file imports to a site file as I expect, and I then interpolate it to
build a raster with

s.surf.rst input=andes5m elev=andes5m tension=40 smooth=10 npmin=50

This displays fine as a raster, showing the seabed topography I expect.

I then try to get GRASS to display this with terrain (aspect) shading,
using the GRASS script below to create a new raster andes5m.shade

shade.rel.sh altitude=20 azimuth=270 elevation=andes5m


This is then displayed using

d.his h=andes5m i=andes5m.shade

This picture is very grainy, the overall impression of shading is OK, but
the map appears to be built of bricks (about 32x32). I use the term bricks
because each has its own shading within its boundary, so pixel is not
appropriate, each brick comprises several pixels. I have tried changing
tension & smoothing parameters, without making much difference.

The output below describes the elevation raster. If anyone can offer ay
advice to help resolve this, I would appreciate it.



Thanks,

  Brent Wood




 +-----------------------------------------------------------------------+
 | Layer:    andes5m                     Date: Fri Aug  8 02:19:48 2003  |
 | Mapset:   PERMANENT                      Login of Creator: baw        |
 | Location: andes                                                       |
 | DataBase: /home/baw/grassdb                                           |
 | Title:     ( andes5m )                                                |
 |-----------------------------------------------------------------------|
 |                                                                       |
 |   Type of Map:  raster              Number of Categories: 255         |
 |   Data Type:    FCELL                                                 |
 |   Rows:         1548                                                  |
 |   Columns:      2268                                                  |
 |   Total Cells:  3510864                                               |
 |        Projection: Latitude-Longitude (zone 0)                        |
 |            N: 44:05:23.28S    S: 44:14:40.56S   Res: 0:00:00.36       |
 |            E: 174:22:53.04W    W: 174:36:29.52W   Res: 0:00:00.36     |
 |   Range of data:    min =  -1470.000000 max = 0.000000                |
 |                                                                       |
 |   Data Source:                                                        |
 |    site file andes5m                                                  |
 |                                                                       |
 |                                                                       |
 |   Data Description:                                                   |
 |    generated by s.surf.rst                                            |
 |                                                                       |
 |   Comments:                                                           |
 |    tension=40.000000, smoothing=10.000000                             |
 |    dnorm=0.003973, dmin=0.000050, zmult=1.000000                      |
 |    segmax=40, npmin=50, rmsdevi=3.464404                              |
 |    zmin_data=-1470.000000, zmax_data=-485.000000                      |
 |    zmin_int=-1470.000000, zmax_int=-489.320737                        |
 |    @@t                                                                |
 |    ]@(@@t                                                             |
 |    ]@$                                                                |
 |                                                                       |
 +-----------------------------------------------------------------------+