[GRASS-user] Level LiDAR data to gps points

[Shane Carey]

Excellent, that is the way to do it!

Thanks

On Wed, Dec 27, 2017 at 11:14 PM, Vaclav Petras <wenzeslaus at gmail.com>
wrote:

> [cc'ing the list]
>
> One way to do the spatially variable shift is to do what I suggested
> earlier, but instead of the last step (computing average), interpolate a
> surface from the differences at each point using, e.g., v.surf.rst with
> zcolumn option set to the difference column computed earlier. For the
> average, the assumption was that the shift is everywhere the same. Here,
> the assumption is that you can interpolate in between the points, i.e. that
> the interpolated values are meaningful. This assumption might be broken
> e.g. when some abrupt surface change influenced the vertical shift of the
> lidar points in a specific area/around certain GPS point (unlikely case).
>
> To actually shift the data, you need to either do the calculation in the
> attribute table (using the aforementioned modules such as v.db.update) or
> more simply, and likely more efficiently, apply the shift to a raster with
> binned elevations from r.in.lidar using r.mapcalc (same expression as
> before, just with a raster map name instead of a constant).
>
> On Wed, Dec 27, 2017 at 5:22 PM, Shane Carey <careyshan at gmail.com> wrote:
>
>> Oh super Vaclav, is there a way of doing a spatially variable shift or
>> can you only do a block shift?
>>
>> Thanks for your help :-)
>>
>> On Wed, Dec 27, 2017 at 4:42 PM, Vaclav Petras <wenzeslaus at gmail.com>
>> wrote:
>>
>>> On Tue, Dec 19, 2017 at 3:29 AM, Shane Carey <careyshan at gmail.com>
>>> wrote:
>>>
>>>>
>>>> I have some LiDAR data that was not levelled properly and I was
>>>> wondering does anybody know of a way of levelling the las files to gps
>>>> points?
>>>>
>>>>
>>> Hi,
>>>
>>> if the issue is really just vertical (which is usually the case), you
>>> can use v.transform zshift to shift by the given value. For rasters, e.g.
>>> DEM based on binning the lidar points with r.in.lidar, you can use
>>> r.mapcalc to shift it (e.g. expression "new = old + 2").
>>>
>>> The shift can be determined by binning (or even interpolating) the
>>> points, followed by querying raster using the GPS points as a vector using
>>> v.what.rast, and then by computing the difference using v.db.addcolumn and
>>> v.db.update. Finally, you can average the difference using v.db.univar. One
>>> (average) values will be usually good enough for lidar point cloud, for UAV
>>> point cloud, spatially variable shift might be needed.
>>>
>>> Vaclav
>>>
>>>


-- 
Le gach dea ghui,
*Shane Carey*
*GIS and Data Solutions Consultant*
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