[R-sig-Geo] decimate points from a Lidar file

[Jonathan Greenberg]

Michael:

    Typically Lidar files come "tiled", e.g. you get a set of point 
clouds pre-subsetted to, say, 1km x 1km window.  These subsetted data 
are usually a LOT easier to write some batch scripts to deal with than 
trying to analyze the entire database at once.  Have you seen if the 
source of the Lidar data has a tiled set (most companies will provide 
the data this way)?  This will expand the number of things you can do 
with the data...

--j

Michael Denslow wrote:
> Dylan, Ken & Roger,
>
> Thank you for the suggestions and advice. This gives me a great place to start.
>
> Michael
>
>
> These are all
> On Tue, Sep 8, 2009 at 2:36 PM, Roger Bivand<Roger.Bivand at nhh.no> wrote:
>   
>> On Tue, 8 Sep 2009, Dylan Beaudette wrote:
>>
>>     
>>> On Tuesday 08 September 2009, Michael Denslow wrote:
>>>       
>>>> Dear R-sig-geo,
>>>>
>>>> I am posting the following question for one of my professors. I would
>>>> appreciate it if you could help me point him in the right direction.
>>>>
>>>> He is looking for a function which could take a lidar point shapefile
>>>> and decimate (remove) a selected number of points
>>>> from the file. Below is what he asked me. The file of the merged LIDAR
>>>> tiles contains roughly 350,000,000 points.
>>>>
>>>> "I am looking for some software to take a shapefile of lidar elevation
>>>> points and remove a selected number of points. The shapefile
>>>> (with fewer points) would then be used to generate a DEM. A
>>>> calculation of the appropriate resolution for a DEM can be made based
>>>> on a relationship between the
>>>> number of points and extent of the study area. The goal is to create
>>>> DEMs at a series of resolutions with the appropriate points to area
>>>> relationship. So a
>>>> software program with which you could remove a selected number of
>>>> lidar points would be helpful."
>>>>
>>>> I imagine that one of the packages that deals with point data (e.g.
>>>> sp, spatstat) can do this kind of thing but I am not sure how to get
>>>> him started. Can anyone point me to a specify function or make a
>>>> suggestion.
>>>>
>>>> Thanks in advance,
>>>> Michael
>>>>         
>>> Hi,
>>>
>>> I would recommend the r.in.xyz command within GRASS GIS. It can be used
>>> for
>>> operations very similar to what you are looking for. The resulting file is
>>> a
>>> raster, decimated to some set resolution. Some screen shots:
>>>
>>> http://hamish.bowman.googlepages.com/grassfiles#xyz
>>>       
>> Right, it is not possible to read 350 M points into R and do anything with
>> them. I would actually suggest using a database as a store directly,
>> possibly with some spatial support, but not necessarily. If the data were,
>> say, in a database, subsets could be read in by selecting, or sample
>> inclusion fields could be added to a table for samples, giving the FID
>> values to be chosen. I feel that this would give the flexibility needed.
>> Both SQLite and PostgreSQL are supported by OGR, so may also be accessed
>> from rgdal functions if the appropriate drivers are present, but dumping to
>> CSV files for smaller samples ought to be OK. If you are on Windows, see
>> OSGeo4W or FWTools for ideas. The advantage of a database over a flat text
>> file is that selection is much easier.
>>
>> You could also look at the SQLiteMap package, which uses an alternative set
>> of spatial extensions for SQLite. Import the data outside R, then select
>> from inside (untried).
>>
>> Hope this helps,
>>
>> Roger
>>
>>
>>
>>     
>>> Cheers,
>>> Dylan
>>>
>>>
>>>       
>> --
>> Roger Bivand
>> Economic Geography Section, Department of Economics, Norwegian School of
>> Economics and Business Administration, Helleveien 30, N-5045 Bergen,
>> Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
>> e-mail: Roger.Bivand at nhh.no
>>
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>>
>>     
>
>
>
>   

-- 

Jonathan A. Greenberg, PhD
Postdoctoral Scholar
Center for Spatial Technologies and Remote Sensing (CSTARS)
University of California, Davis
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The Barn, Room 250N
Davis, CA 95616
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