Adjustment of LIDAR based on Survey

[boberdorf]

Often I obtain LIDAR data for concept studies and conventional survey over a small portion of this.  The LIDAR typically covers the broader area and is used for broader studies, catchments, optioneering and concept design.  The survey may either be existing survey (for other projects) and provided with the LIDAR, or obtained subsequently specifically for detailed design.

I find that whilst the LIDAR looks relatively correct, that when compared to the more accurate conventional survey there are often discrepancies. Often the supplier of the data is not familiar with datums or the history of the data, which may also be the cause of some of the discrepancies.

If the discrepancy is relatively uniform, it is relatively straight forward to adjust the LIDAR data.  However often this is not the case.  Is there a method to "adjust" the LIDAR to more accurate survey data?  I imagine it would be similiar to rubbersheeting of aerial imagery, but in three dimensions.

I appreciate that the LIDAR data will never be as accurate, but even if it just looks better I would be happy.

Thanks

[rl_jackson]

"I appreciate that the LIDAR data will never be as accurate, but even if it just looks better I would be happy."

That really depends on the type of LIDAR data you acquiring and where it's coming from or how it was done. LIDAR data can technically be provided by Laser Scanners, Low-Level Flights, Standard Flights and Satellite (among others).

From a design/transform prespective I would think that your best bet would to use an average elevation method to adjust the data to the existing survey, as anything else would skew that data. You should also have multiple surface files one of the Existing Conditions, one of the LIDAR and one of the two together, which you could add additional points to surface to help bring the two together, based on local knowledge of the area or soils information which provides slope infromation based on the particular group. If the Civil Engineer (Designer) needs Tc flow paths to determine Q etc..., then he would use the LIDAR initially for the overall catchments for rough quatities, and then move to the existing/LIDAR for subcatchments.

Does this mean you cannot transform it so to speak, I think you could by change UCS and tilting the LIDAR to the Survey, but the effects of this would be far reaching as you move farther away errors would start compounding things just as it would horizontially in a survey (1 second of error in 100' is not the same as 1 second of error in a mile). By doing any form of transform you would then be upsetting the data set you have aquired from its original intent, at which point it becomes your data, and that sounds extremely dangerous if the question ever came up as to how the data was adjusted.

In any situation I would have a big note on the drawings regarding where the datasets (Existing & LIDAR) came from, and if I had to go so far as state additional points were added to the perimeter of the the Existing Survey, I would state that as well.

As a surveyor I once worked with said to me:

"Give credit were credit is due."

[ToddRogers-WPM]

Lidar has come a long way in the past few years.  It's actually pretty accurate.  I imagine the data you're getting is probably processed in "surface" coordinates or grid.  What is surveyed in grid is actual lengths.  For example, if an alignment of a road is 1000' feet long (in grid), ground coordinates will be different because of the shape of the earth.  I'm not very good in explaining this.  To better understand how this works, there is a great article you can read by clicking the link below...

http://www.amerisurv.com/PDF/TheAmericanSurveyor_Billings-GroundVersusGrid-LDPpart1_Vol10No9.pdf

[boberdorf]

Thanks Todd and rl_jackson. I take both comments onboard and note I do many of the items you noted as routine.

To clarify, I often receive the Lidar data without any details of its origin, datum, accuracy or even date! Usually my clients are project manager types with little understanding of these issues.

It would be useful to be able to "transform" the data in a more rigorous manner than block shift, rotate and scale manually.

[antoniovinci]

As far as I know, there's no way to "fine" tune the position of a point cloud, changing its inner coordinates.

You can transform it from datum A to datum B, but you won't move it "X feet to Easting" and "Y feet to Northing" (unless you make it a block).

I guess your Lidar file doesn't fit exactly to your survey, because of the GPS precision of that Lidar catch: may you copy and paste here the metadata of the flight?

[autoMick]

I'm not sure if this is what you are after, but if you are trying to vertically warp a lidar surface to match ground survey points you could do this.

1. Create LiDAR surface

2. Import survey points (say there are 50 of these for argument's sake)

3. Obtain elevation difference between the 50 points and the LiDAR surface

4. Make a surface of those elevation differences (using only the 50 locations to create the surface). The surface extrapolates the elevation differences between the ground survey points.

5. Now add/subtract the differences surfaces from the LiDAR surface

This in theory gives the detail of the LiDAR surface, but adjusted to match the ground survey results.

Does this help?

Cheers

- Mick

[MikeEvansUK]

I have done this in the past with some good results.

Usually if you have a 3d survey you just need a hole cut in the lidar then paste the two together.

As others have stated Lidar is fairly accurate now (depending on filtered or non filtered) but not as good as GPS surveys etc.

I would usually do the following.

Import the two surveys and perform a volume surface analysis. Note: this mar require the converting of one surface from Grid to Tinn etc. See later for workaround.

Once a volume surface is generated it is easy to see the differences in the properties, so you can just raise / lower the Lidar to suit.

Next trim the Lidar data so you have a hole where the other surface overlays, I would ensure a small boundary of 20m (depending on the grids used) is included so it will blend the two surfaces together. Add the two surface boundaries as breaklines before any pasting is done.

So to convert the surface (if required) to a Grid surface I always generate surface labels at a grid (a small grid or to match the lidar) with a 3dpoint as the marker. Explode and use dataextraction or convert cogo to create a grid surface for use elsewhere.

Try not to trim where ditches and other features reside, chose the trim locations wisely and always check contour results after combining.

[jlavoieQKYXY]

Hey Mike, I know this was a post from a few years back but I'm having a similar problem I would like to try to automate a fix for. 

I have a drone survey and we have GPS audit points and I want to adjust our drone survey to our audit points (elevation wise as N and E both match) 

I want to know if you have come up with a better way of editing the surface now since a couple years have passed, if not would you be able to explain step 5 a little bit more for me? 

How exactly would you add the elevation difference from one surface to another? also my audit points are only on the inside of the site but I can add my site outer boundary to those points to make the surface extrapolate to the entire site. 

I'm using Civil 3D 2016. 

[nonbeard13]

Welcome to the forum

what mr. evans is saying check the area between your two surfaces - the gap being filled when the surfaces are combined (merged)

if you have retaining walls, roads (with embakment), rail roads (with embakment), drainage channels - make sure the contours do not block the natural slope or create a large bump (elevation change) that would not be realistic. 

usually i use the survey point surface boundary and offset by 50 - 100 feet, depending on how steep or how many roads/channels cross the boundary.

then i merge the surfaces and let Civil 3D generate the tie between the two surfaces.

and now the fun begins

start at one point along the survey surface boundary and look at the contours; do the contours make since?

I have walked too many topo surveys and just turn on the tin lines in the surface and make manual adjustmenets as needed.\

you may have to add points, break lines or polylines (to be used as contours) to make the combined surface look realistic.

it is not a speed method, but it works and gets a better looking surface. 

the other option - plot the combined surface and walk the site.

be prepared for the weather and owners and animals, too.

best of luck

nonbeard13