[R-sig-Geo] Substracting DTM from Terrestrial LiDAR point cloud

[Nathalie Morin]

Hello Edzer,

Yes ! Super ! It worked ! Thank you so much for your suggestion. I converted
the SpatialPoints object into object of class SpatiaPointsDataFrame. The
function over() worked and I was able to substract the 2 Spatial*DataFrame's
attributes, i.e., z from the TLS point cloud from the z of the DTM to obtain
TLS canopy height. Thanks also for making me discover the r-sig-geo mailing
list. Very efficient...!

Cheers,
Nathalie

-----Message d'origine-----
De : r-sig-geo-bounces at r-project.org
[mailto:r-sig-geo-bounces at r-project.org] De la part de Edzer Pebesma
Envoyé : mercredi 10 octobre 2012 11:32
À : r-sig-geo at r-project.org
Objet : Re: [R-sig-Geo] Substracting DTM from Terrestrial LiDAR point cloud



On 10/10/2012 06:00 PM, Nathalie Morin wrote:
> Hello !
> 
>  
> 
> I am working with georeferenced LiDAR data with R and sp. I am trying 
> to normalize height and remove the slope effect from a Terrestrial 
> Laser Scanner (TLS) point cloud.
> 
> The available data is :
> 
> -          a TLS point cloud in ASCII format converted to sp objet of
class
> SpatialPoint (variable named "tls.z.sp")
> 
> -          a raster DTM in TIF format (1 m resolution) converted to sp
> object of class SpatialGridDataFrame (variable named "tls.dtm$z")
> 
>  
> 
> The method consists in subtracting the DTM z value from each TLS point 
> z value. I proceeded to some tests with the "over()" function with :

Where is your TLS z value? If tls.z.sp is a 3D SpatialPoints object, I'd
suggest to make it a 2D object with z as the attribute value, meaning it is
a SpatialPointsDataFrame object.

> 
> x = "SpatialPoints"
> 
> y = "SpatialGridDataFrame"
> 
>  
> 
> nz <- over(tls.z.sp, tls.dtm$z, tls.z.sp-tls.dtm$z)

Hard to see, but maybe:

nz = over(tls.z.sp, tls.dtm)

then, if tls.z.sp is a SpatialPointsDataFrame with z as attribute:

tls.z.sp$z - nz$z

gives you the differences you want, and

tls.z.sp$difference = tls.z.sp$z - nz$z

adds them as a variable to the SpatialPointsDataFrame oject.

> 
>  
> 
> I am not sure to be on the right track. Any suggestion would be very 
> much appreciated !
> 
>  
> 
> Cheers,
> 
> Nathalie
> 
>  
> 
> 
> 	[[alternative HTML version deleted]]
> 
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> 

--
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster Weseler Straße
253, 48151 Münster, Germany. Phone: +49 251 8333081, Fax: +49 251 8339763
http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de

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