LiDAR images

[Chasbo]

Has anyone got any experience of interpreting LiDAR images?

 

I know this request is not exactly canal related but my garden is at the western terminus of the Shropshire tub boat canal and a friend has given me a copy of the LiDAR image showing the area around my house.

 

The image shows an “anomaly” at about the position where it is thought one of the shafts of the tunnel and shaft system should be. If this anomaly is caused by the shaft then the laser must be able to penetrate at least 12 inches of crushed stone plus whatever else is capping off the shaft. Surely this is not possible?

 

The image can be found www.oldwynd.co.uk/lidarimages.doc

The anomaly is the black dot half way down the image on the left hand side just to the north east of a hollow.

 

More information at www.oldwynd.co.uk/history.html

 

[Paul's Nulife4-2]

My understanding,is that it is not ground penetrating, it only maps 'actual or hard targets'.

Similar portable units have been used for some years in the military , its basically a very accurate 3d ish mapping tool.

So if an object is big enough to reflect a laser,it will map it.

Maybe some models can be linked with ground penetrating or Geo Phis but that gives a very different image to interpret.

That's just my take in it anyway.

Edited December 14, 2013 by Paul's Nulife4-2

[John Williamson 1955]

As it's only a very small image as linked, I can't be sure.

 

It might be something that absorbs the laser light, maybe the vegetation there is being affected by a difference in moisture levels?

 

Looking at other LiDAR pictures on the web, they show black where there is standing water, so maybe a large puddle or small seasonal pond?.

[Chasbo]

As it's only a very small image as linked, I can't be sure.

 

It might be something that absorbs the laser light, maybe the vegetation there is being affected by a difference in moisture levels?

 

Looking at other LiDAR pictures on the web, they show black where there is standing water, so maybe a large puddle or small seasonal pond?.

 

Thanks for looking John.

No, there is nothing visible there on the surface. no vegetation, no water just part of the drive outside my house.

 

My understanding,is that it is not ground penetrating, it only maps 'actual or hard targets'.

Similar portable units have been used for some years in the military , its basically a very accurate 3d ish mapping tool.

So if an object is big enough to reflect a laser,it will map it.

Maybe some models can be linked with ground penetrating or Geo Phis but that gives a very different image to interpret.

That's just my take in it anyway.

Hi Paul, That is what I thought as well. The first image shows the area with vegetation and the second one shows the same area with not just vegetation removed but also buildings and even a bridge removed. How can it do that unless the laser can penetrate bricks and tiles.

[Theo]

LiDAR is like Radar but uses laser light pulses. LIght Detection And Ranging. It can be used to "remove" masking vegetation as shown by comparing the upper and lower images. As Paul says I can't see how it can penetrate the ground. Ground penetrating radar would be needed for that.

 

Nick

[RLWP]

A couple of useful LiDAR resources:

 

http://www.lidar-uk.com/what-is-lidar/

 

http://www.lasermap.com/laserM/en/doc03.htm

 

Based on the second one, I wonder if the bridge is missing because it isn't in the dataset - it got missed in the scan - not because it was 'removed'

 

Richard

[Paul's Nulife4-2]

Thanks for looking John.

No, there is nothing visible there on the surface. no vegetation, no water just part of the drive outside my house.

 

 

Hi Paul, That is what I thought as well. The first image shows the area with vegetation and the second one shows the same area with not just vegetation removed but also buildings and even a bridge removed. How can it do that unless the laser can penetrate bricks and tiles.

Ime guessing,that because it produces a digital bit map,the software can remove or manipulate images,extrapolating the digital footprint local to the given target area,& use that information to fill in the blanks, joining areas of equal or similar reflected densitys. a bit like photo shop but with much more bells ,whistles & finery.

But as I say,its a guess.

[RLWP]

Ime guessing,that because it produces a digital bit map,the software can remove or manipulate images,extrapolating the digital footprint local to the given target area,& use that information to fill in the blanks, joining areas of equal or similar reflected densitys. a bit like photo shop but with much more bells ,whistles & finery.

But as I say,its a guess.

 

That would make sense. Something like 'remove anything with a small footprint that is more than ten foot taller than most of what is around it' would remove trees, hedges, buildings

 

Richard

 

MORE: I wonder if that is actually a gap in the dataset. It is the only black bit in the data

Edited December 14, 2013 by RLWP

[Chasbo]

A couple of useful LiDAR resources:

 

http://www.lidar-uk.com/what-is-lidar/

 

http://www.lasermap.com/laserM/en/doc03.htm

 

Based on the second one, I wonder if the bridge is missing because it isn't in the dataset - it got missed in the scan - not because it was 'removed'

 

Richard

Thanks for those links Richard.

I think the second link gives me the answer - that the software is too clever for its own good.

 

 

 

 

I wonder if that is actually a gap in the dataset. It is the only black bit in the data

 

I wondered that too. But it is too much of a coincidence that the gap in the data is just where the shaft is.

 

There are other black areas on the rest of the image ( that I did not include) but these are all heavy shadow due to a steep gradient.

[RLWP]

The second link does confirm that LiDAR is a tool that produces suggestions, not conclusions.

 

If you have searched the ground and there is nothing, the LiDAR results are wrong

 

Richard

[John Williamson 1955]

You say the black dot is on your drive. Where was your car parked when the image was being taken? Depending on the angle of the laser and your car roof, it could have reflected the light away from the sensor, giving the same black effect as standing water.

[magpie patrick]

All these systems have potential for shadows, resulting in gaps in the data, they can also be fed into models that can interpolate reasonably well and extrapolate less well. Most of the desk based work I do has a lidar survey in it somewhere.

 

Lidar is not ground penetrating but can be confused by soft or shiny surfaces, it doesn't record water as level but gets very confused depending on the reflection back. You need to have a good idea what the lidar image is looking at to interpret it correctly.

 

Whilst it can't penetrate ground, neither can aerial photography but that picks up a lot that can't be seen at ground level, so don't dismiss whatever it is, just treat it with caution

[trackman]

Can't get either link to work on my phone so it's hard to assist. However, this kind of survey often uses more than one geophysical method, linking the results.

Old shafts like this usually show well in images produced by ground proving radar, for example. Are you sure this survey didn't make use of some other method besides the Lidar?

[Chasbo]

Thanks everyone for your thoughts. I will make some enquiries tomorrow to find out, if I can, exactly when the image was taken and what if any other techniques were involved. If the LiDAR had shown that the shaft was near the surface and had not been properly capped or filled it would involve me in great expense. (How much would it cost to fill a hole 120 feet deep and 10 foot diameter!)

 

CB

[John Williamson 1955]

The only signs of the shaft easily visible from the air using normal aerial photography that you wouldn't see easily from the ground would be vegetation such as grass growing at a different speed to its surroundings due to either more or less moisture. It's easier to see in infra red and a number of previously unknown archaeological sites have been found this way.

 

What LiDAR is good at it looking through overlying vegetation at the actual ground levels so you can see things like hidden ditches, and at the scale of your images as I can look at them, there's nothing untoward apart form the black marks, which are normally signs of something reflective such as water, glass or a similarly glossy paint finish on the ground. The image shows black because the(non) return times out at the receiver. It would be interesting to see a normal aerial picture taken at the same time as the LiDAR images.