Messages

16

General / Corridor Mapping

« on: December 30, 2012, 09:53:26 AM »

Hi
This has been touched on in some of the posts. I have only done project of this nature and it dit not work very well. I have noticed that square project process well with no problems , but as soon as you have a long slim rectangular project the problems start.
To use both the camera positions and the GCP's seems to help a lot.
Any idea's or experiences with corridor surveys that has worked well.
Has anyone tried to introduce tie strips?
Regards
Herman

17

General / Re: LAS files with a shift

« on: December 30, 2012, 09:47:32 AM »

Hi Matt & Willis
Thanks for the reply. I will try with a different projection. But I would eventually have to revert back to my local projection.
I think the problem is not with LAStools.
The PS file and the LAStools file (after classification) plot in exactly the same position, so LAStools does not intorduce any positional error
I think the problem is partly a Globalmapper problem and a PS problem - the DEM and TIFF files are correctly located in Ver.8 of GM but the LAS file is offset by 80km's.
But since ver.11 you have to multiply with -1 to have data plotting the correct way up , TIFF images plot 180deg. rotated etc.
I agree Matt the bit I have seen and the attitude shown by Martin makes me very keen to invest in this software and I will gladly pay to use it.
Similarly the PS guys support is also great and hopefully this is only a minor projection offset error.
Have any you tried VRmesh , also looks very promising.
Regards
Herman

18

General / Re: LAS files with a shift

« on: December 29, 2012, 07:22:02 PM »

Hello Alexey

Thank for the quick reply - I have to note again that between yourself & Dmitry support is very professional and your software is great!!!!!!

I unfortunately I only work in Schwarzeck , I will try with a smaller project and convert the GCP to
WGS 84 and test that.

Regards and the best to you all for 2013

Regards
Herman

19

General / Re: Trees!

« on: December 28, 2012, 09:23:01 PM »

Hi

I have experimented with the LASTools and the results looks very promising.  Trees, and buildings where very successfully removed. It is semi desert terrain and a mining operation , but still very
impressed with the ease and the speed.
I only struggle with a huge shift in the LAS data, this seems to originate in PS , not in LASTools.
Would appreciate any possible explanation for this

Regards
Herman

20

General / LAS files with a shift

« on: December 28, 2012, 09:09:49 PM »

Hi
I am a surveyor working mostly in Namibia and we have a rather standard projection.
A Transverse Mercator , two degrees wide per panel and based on the the odd longitudes
lines e.g. 13, 15, 17 Deg. East .
Our Northing's  is based are base on the 22 deg. South latitude and we use the Besels 1841
Geoid
The system is called Schwarzeck and for example the the Lo panel 15 is the standard
EPSG : 29375 projection - catered for and working perfectly in PS.
The problem I do have is that the LAS files generated are positioned roughly 1 deg. ,
85km to 110km wrong when viewed in Global Mapper and overlaid with the TIFF
images or the DEM that all plot perfectly.

Can anyone offer a possible explanation?

Regards
Herman

21

General / Re: Once again about DEM and 3D accuracy

« on: December 28, 2012, 08:56:55 PM »

Hello George

Sorry for replying so late, yes I get results in the order of xy= 1 * GSD & z = 2 * GSD
In the example attached the area was flown at 2.5cm GSD and the results are shown.
Points were tested with conventional survey and heights where within 40mm.
Regarding smooth vs sharp , I think is has got to do with what algorithms are use in
the processing , sharp being more suited to to built up areas.


Hello Piste
In Photoscan I obtain a total error of 0.014 m and 0.284 pixels using 14 GCP with known coordinates...
very good I say...(pixel size was of about 3cm)
When I have compared the point cloud exported with that one obtained in the same area
with a terrestrial laserscanner I have found an error in height till 50 cm...
Why this?
Can be you be assured that both surveys are based on the same height control?
I have done a 1000ha survey and the previous baseline survey was done by lidar,
Comparable results where good , I can forward some images, they are to large to attach,
if you are interested email me at    -  herman@sasurv.com
 
Hello Santiago

The report said my error was 0.497 pix and my ground resolution was 0.077 m/pix. I asked the support team to explain the results to me but their explanation didn't clarify my doubts. They told me an error <0.5 was considered to be good, but I want to know what's the error in meters for my project. Or compared to a total station or a RTK GNSS how did I do. Do you know based on what do they say <0.5 is good.

The 0.497 pix basically indicate that in generating the point cloud and in aligning the photographs
image to image the GCP marked in photo 1 agrees to photo 2 at an average of 1/2 a pixel.
The 0.077m is the average combined xyz error , this can be split in the actual x,y & z error per
point "View errors" tab in "Ground control" view
You can accept this accuracy but results are still based on the number and also position of GCP.
To few GCP and to far apart would have the obvious results.

Hello Eduardo

What in your opinion would be the right strategies for corridor mapping

I dont really know , I have just done a 65km pipeline survey. The results were very bad.
I have noticed that a slim rectangular shape does not process well. As soon as the ratio of
the long side related to the short side get to approx. 0,2 the results are poor.
This can firstly be spotted if the plane of the photo/camera's are not parallel to the bounding
box.
In a slim corridor the area rotates very easily along the length of the corridor.
In the 65 km pipeline the area was shaped in a "C" form and I thought this should give it
strong geometry , but no luck.
We had 380 GCP spaced at a 250m interval, and we opted to cut the corridor into 65 lenghts
of approx.  1.5km in length (with some overlap) and this worked ok.
I don't know really what approach to take, maybe tie strips flown perpendicular to the corridor
might help, this would result in a lot of extra work but might be essential , will experiment with
this in the next project.

22

General / Re: Once again about DEM and 3D accuracy

« on: November 08, 2012, 09:40:23 PM »

Hello George
I have done a number of practical surveys and have compared results with Totalstation and GPS surveys. The total station and GPS survey was based on the same control points used to fix the ground control points , thus the surveys are inter related.
A few spot checks was done as follows. The survey was draped over the cadastral (legal) boundaries captured independently but related to the same co-ordinate system and it is a perfect match. This being verified by seeing the boundary lines intersect the cairn marking the the boundary beacon or with walls and fences perfectly fitting the boundary layout.
Regarding test done by surveying a GCP but not using it to control the the PS project, results are as follows. In theory I understand that accuracy should be equal to XY=2*GSD and Z=4*GSD. I find it rather to be in the range XY=1*GSD and Z=2*GSD. When testing in relation to a arbitrary spot shot taken , not a physically identifiable point, results are  better than  XY=2*GSD and Z=4*GSD.
Basic rules of interpolation should be applied and rather too many GCP's.
Also the shape of the project seems to play a roll , long thin project (corridor mapping) is problematic if approached in the wrong way.
Sampling rate has a definite influence on accuracy , but for your standard 25min flight with 500-600 images ,options "smooth" and "high" produce the results above. I fly with an average GSD of 4-5cm.
Regards
Herman