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Discussion: LiDAR advice

in: Orienteering; General

Oct 17, 2015 5:21 PM # 
Wil Smith and I are looking to get LiDAR flown for map development. What specs should be we requesting from the company for developing good O maps? The terrain is quite flat. lots of conifers and open rock.,-80.087160...

Thanks for any advice.
Oct 17, 2015 5:51 PM # 
I'm a bit hesitate to say much here because I don't know much about it. But, I was in on the conversations about specs for the NAOC 2016 maps and picked up some things.

1. It is desirable to have at least 1 GROUND point every sq. meter. However, depending on the tree canopy, you may have to ask for a denser sampling because some of the shots will never reach the ground. Eddie did an analysis based on the previous lidar data for Oak Hill at Dartmouth and concluded that, in some areas, only about 25% of the lidar actually reached the ground. (or I think that was the conclusion).

2. Higher density lidar will cost more for the data but could save time/money on the field checking end. In any case, the minimum desired is at least 1 GROUND point per sq. meter.

3. Get the lidar during leaf-off and without snow cover.
Oct 17, 2015 6:06 PM # 
I want to add another thing to factor in and that is the ability of your field checker. If you are using volunteers with limited field checking experience, than you are probably better off with better lidar so that there will be fewer field corrections necessary. On the other hand, if you have an experienced field checker, they should be able to add in the features that poorer quality lidar doesn't pick-up on.
Oct 17, 2015 11:23 PM # 
Mmmmm, Shield...
Oct 17, 2015 11:28 PM # 
I don't think SHIELD does LIDAR.
Oct 18, 2015 7:29 AM # 
I wouldn't give LIDAR data to an inexperienced field checker. They may believe it.
Oct 18, 2015 9:00 PM # 
Big Jon:
Not sure this is much help but Lidar data in Scotland has very high point density - I've heard 15cm accuracy mentioned. Certainly plots can have 50cm contours. The Difference between DTM and DSM is also critical - it gives a vast amount of location info - clearings, different heights of trees etc. Difference in the DTM can also pick out even small paths - as grey lines marking the edge of them.
Oct 18, 2015 10:23 PM # 
It may be 15cm accuracy but can still be 1 metre resolution as it is around here. Lidar of this accuracy picks out many overgrown and invisible old paths as well.
Oct 19, 2015 4:18 PM # 
Ultimately the lower point spacing (average distance between points) you get the more info you can get from the data. Theoretically the contour interval you can extract has nothing to do with the point spacing but the problem is that with higher point spacing you are more likely to miss small details like little depressions or knolls because they may fall between points.

As for what to ask for: make sure to be clear that you want the full set of data and not just first, last, or ground returns. You want an las file with fully classified data. Apparently you can classify the data yourself but I wouldn't know how to do so and I gather it's a painful process.

Lastly it may well be worth asking if they can take orthophotos with the same flight so you know you have up to date matching data. No idea how much extra that might cost.
Oct 19, 2015 4:51 PM # 
Hammer - I was impressed with a presentation by a Sumac rep (consultants out of Thunder Bay) that I saw last week at the Toronto 2015 ESRI Users Conference. Might be much lower cost alternative for a bare earth DEM, with a significantly shorter turn around.
Oct 21, 2015 1:39 AM # 
Bender - looks like a similar process to SGM (semi-global matching). We had a couple of areas done with this method, although we used a traditional fixed-wing aircraft for the photography because we had too many dramas trying to get a UAV to do it.

The resulting bare-earth DEM was very good, but it didn't include any of the rather large boulders - I assume they got filtered out in the process of removing all the tree returns. Deliverables were the SGM DEM and a ~5cm orthophoto, which I then used to trace all the rock detail I could find, and the mapper used those scribblings plus 2.5m contours as his base map. Feedback from the mapper was that it was more than adequate, and he certainly ripped through the area in very fast time.
Oct 21, 2015 1:39 AM # 
Some operators gather digital photography at the same time as the LIDAR to assist with their data classification process. This may not be as good as full orthophoto imagery, but could be useful if available - worth asking about.
Nov 3, 2015 11:36 PM # 
Yes we would be using professional mappers for this. Thanks for the advice so far but if there is anybody that has extra advice we would welcome that as well. ie., what exactly should we be saying to the company. Here is the area flown but we want....this that and what else.

We already have lots of information on how to tape control descriptions onto our arms but are keen on extra info on map making advice. Thanks everyone.
Nov 4, 2015 12:27 AM # 
I can think of several people who I think would be well-qualified to speak this to this question, but I haven't seen them weighing in yet...
Nov 4, 2015 2:36 AM # 
AP isn't the best platform for detailed technical discussion that is frequently dependent of how you want to use the data and exactly what the terrain is. A discussion with one of the experts is probably best. I'll follow up to you with an email and feel free to ask me any detailed questions that pertain to your exect situation that way or via phone if necessary.

My base recommendation is to refer any company flying lidar for you to the USGS standards, and let them know you want QL1 quality data, delivered as a classified point cloud, but that you don't need the QA work, or the ground reference stations, or manual breaklines. Hydro enforcement, and concurrent 4 band photos would be valuable add ons if within budget.
Nov 4, 2015 2:47 AM # 
OK, there we go.

(I don't think he was looking for a technical discussion, he was looking for an expert. Mission accomplished.)

This discussion thread is closed.