Assigning a standard CRS to given point clouds whose XYZ values represent arbitrary values

[gama]

Hello...

I have a photogrammetrically generated point cloud with respect to arbitrary reference system. Therefore, X,Y,Z coordinates of the points represent both negative and positive values. I want to assign a standard reference system (say WGS84) to the point cloud that I have. Could you please let me know whether this can be done with LasTools.

Regards

Gama

[Martin Isenburg]

Hello Sanka,

you have a point cloud with an "arbitrary reference system" and want to go to a standard one? Well ... you will have to have some control points whose positions are well known so you can "snap" the point cloud to a proper geo-referencing? I believe that CloudCompare has functionality that allows you to set up such a correspondence to known points and then move the entire point cloud accordingly. Until your points *are* already transformed into some known reference system, LAStools cannot help you ... |-:

Martin @rapidlasso

--
Download LAStools at
http://lastools.org
http://rapidlasso.com
Be social with LAStools at
http://facebook.com/LAStools
http://twitter.com/LAStools
http://linkedin.com/groups/LAStools-4408378
Manage your settings at
http://groups.google.com/group/lastools/subscribe

[Terje Mathisen]

Martin Isenburg wrote:
> Hello Sanka,
>
> you have a point cloud with an "arbitrary reference system" and want
> to go to a standard one? Well ... you will have to have some control
> points whose positions are well known so you can "snap" the point
> cloud to a proper geo-referencing? I believe that CloudCompare has
> functionality that allows you to set up such a correspondence to known
> points and then move the entire point cloud accordingly. Until your

> points *are* already transformed into some known reference system,

> LAStools cannot help you ... |-:

Martin is (as usual) correct, you do need a known reference system
before the absolute (x,y,z) position values can mean anything in the
real world.

If you just need approximate (at the 20-50 cm level) accuracy you can
probably do quite well with just 3-4 reference points per LiDAR tile,
picked up by using any existing georeferenced source like a government
map or even Google Earth: Look for house corners or similar details that
are easy to locate on a map or orto photo, and find the same points in
your point cloud. Personally I would probably start by generate a slope
and/or intensity image from the lidar, and then you
rotate/translate/strech/skew the lidar image so that it matches your
reference image as well as possible, while aggregating all the operations.

In the end this will then become a simple linear matrix operation on
each source point, something like

new_x = x * a[0,0] + y * a[0,1] + z * a[0,2] + a[0,3];
new_y = x * a[1,0] + y * a[1,1] + z * a[1,2] + a[1,3];
new_z = x * a[2,0] + y * a[2,1] + z * a[2,2] + a[2,3];

It is very likely that the Z coordinate will be independent of X and Y,
which means that you can skip four of those multiplications but making
it general will not be significantly slower since a good matrix library
will be able to use SIMD vector ops to parallelize the calculations.

Terje

>
> Martin @rapidlasso
>
> On Mon, Apr 24, 2017 at 9:18 PM, gama <sa...@geo.sab.ac.lk
> <mailto:sa...@geo.sab.ac.lk>> wrote:
>
> Hello...
>
> I have a photogrammetrically generated point cloud with respect to
> arbitrary reference system. Therefore, X,Y,Z coordinates of the
> points represent both negative and positive values. I want to
> assign a standard reference system (say WGS84) to the point cloud
> that I have. Could you please let me know whether this can be done
> with LasTools.
>
> Regards
>
> Gama
> --
> Download LAStools at
> http://lastools.org
> http://rapidlasso.com
> Be social with LAStools at
> http://facebook.com/LAStools
> http://twitter.com/LAStools
> http://linkedin.com/groups/LAStools-4408378
> <http://linkedin.com/groups/LAStools-4408378>
> Manage your settings at
> http://groups.google.com/group/lastools/subscribe

> <http://groups.google.com/group/lastools/subscribe>

>
>
> --
> Download LAStools at
> http://lastools.org
> http://rapidlasso.com
> Be social with LAStools at
> http://facebook.com/LAStools
> http://twitter.com/LAStools
> http://linkedin.com/groups/LAStools-4408378
> Manage your settings at
> http://groups.google.com/group/lastools/subscribe

--
- <Terje.M...@tmsw.no>
"almost all programming can be viewed as an exercise in caching"