[GRASS-dev] How many points in a point layer and v.surf.bspline questions

[Doug_Newcomb at fws.gov]

Hi Folks,

I aggregated all of the bare earth lidar points for the state of North 
Carolina into a single file  Imported all 8.5 billion points for NC into 
one point layer with no topology.  I was sort of curious to see if I could 
generate a seamless 20 ft elevation grid for the State of North Carolina 
from a single data layerusing bspline and  tried the following command:

GRASS 6.5.svn (ncstpft_nad83):~ > v.surf.bspline input=all_nc_be_pts2 
raster=all_nc_be_20ft_bspline sie=40 sin=40 layer=0
WARNING: Coor files of vector map <all_nc_be_pts2 at statewide> is larger 
than
         it should be (158913789952 bytes excess)
Cells for raster map <all_nc_be_20ft_bspline> will be interpolated
ERROR: G_calloc: unable to allocate 18446744071812941729 * 8 bytes of
       memory at dalloc.c:66

This was for a region with the following settings:

projection: 99 (Lambert Conformal Conic)  - North Carolina State Plane 
Feet NAD83
zone:       0
datum:      nad83
ellipsoid:  grs80
north:      1050000
south:      30000
west:       400000
east:       3060000
nsres:      20
ewres:      20
rows:       51000
cols:       133000
cells:      6783000000



I then zoomed into the northeast corner of the state and set the region to 
:
north:      1047363
south:      795102
west:       2505067
east:       3007693
nsres:      20.00007928
ewres:      20.00023875
rows:       12613
cols:       25131
cells:      316977303

and tried again for this area

GRASS 6.5.svn (ncstpft_nad83):~ > v.surf.bspline input=all_nc_be_pts2 
raster=ne_nc_be_20ft_bspline sie=40 sin=40 layer=0
WARNING: Coor files of vector map <all_nc_be_pts2 at statewide> is larger 
than
         it should be (158913789952 bytes excess)
Cells for raster map <ne_nc_be_20ft_bspline> will be interpolated
subregion 1 of 5459

I killed that one and set the sin and sie to 400 rather than 40 .  This 
dropped the regions down to 66.  

GRASS 6.5.svn (ncstpft_nad83):~ > v.surf.bspline input=all_nc_be_pts2 
raster=ne_nc_be_20ft_bspline sie=400 sin=400 layer=0
WARNING: Coor files of vector map <all_nc_be_pts2 at statewide> is larger 
than
         it should be (158913789952 bytes excess)
Cells for raster map <ne_nc_be_20ft_bspline> will be interpolated
subregion 1 of 66

This is about halfway done, so I should see the results in another 1.5 
days or so. 

I have two questions.  First, is there a problem with importing large 
point datasets that is being highlighted by the warning?  I did notice 
that the size of the coords and hist files did not match for the point 
data set.

        Second,  is about v.surf.bspline and sin and sie.  I have seen in 
the documentation that the sie an sin needs to be twice as large as the 
spacing between points as a good starting point.   The density of the 
lidar data sets that make up this ground point layer varies between 1m and 
5 m .   Obviously, making sin and sie larger reduces the number of 
subregions, but how does that affect the accuracy of the surface 
generated?  If I'm trying to generate a 20ft grid, is using sin and sie 
options of 400 absurd?  ( I.E.,  It works faster but the results are less 
accurate)


The version of GRASS used was the weekly snapshot from 12/25/2010.  The 
computer has a 1.6 GHz Xeon quad-core cpu with 16 GB RAM.


Doug




Doug Newcomb 
USFWS
Raleigh, NC
919-856-4520 ext. 14 doug_newcomb at fws.gov
---------------------------------------------------------------------------------------------------------
The opinions I express are my own and are not representative of the 
official policy of the U.S.Fish and Wildlife Service or Dept. of the 
Interior.   Life is too short for undocumented, proprietary data formats.
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