Evidently you have never been in a helicopter piloted by a Vietnam vet...
True!
If I incorrectly determine the height and/or location of an obstruction and an aircraft strikes it and fatalities result, who gets sued?
Everybody with any connection to the project gets sued.?ÿ Comparative negligence will determine how much of the damages get assigned to the surveyor who blew the tree height.
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I consider obstacle mapping of approach paths and airport environs to be pretty serious business. Not only is there a potential for the loss of lives but economic costs associated with aircraft load limits, approach and departure procedures and the like. Clearing obstructions is costly and often legally cumbersome.?ÿ
I would approach the work with the seriousness it deserves.?ÿ
That said, it is interesting to observe that, in the case of instrument approaches, the obstruction/obstacle definitions are made with respect to the end of the runway (or displaced threshold) but ILS procedures on runways with a Glide Slope (GS) are designed to have the aircraft land perpendicular to GS. See attached copy of an approach plate for my local airport.
In the event further clarification is required, the threshold clearance height is the height of the aircraft as it crosses the end of the runway and the GS of 3 degrees is the approach angle to the runway intercept (point perpendicular to the GS instrument).
If the aircraft is so low that it hits the trees then it was going to hit something anyway. Nobody intentionally flies to miss obstructions by 3 ft.
Evidently you have never been in a helicopter piloted by a Vietnam vet...
Doesn't need to be Vietnam vintage.?ÿ Flying knap of the earth is still SOP.?ÿ I'm an air Cav army brat and spent most of my youth living on airbases.?ÿ We used to have cobras fly between our housing at speed with their fuselage below the roof lines.
In TN we have buildings, vegetation, and breaklines from Lidar. Vegetation is always gathered when lidar is flown. It may not be classified or it may be just vegetation or low med and high vegetation. All depending on what the client wants to pay for.
Do I understand you correctly? Two States that you practice in have freely available LiDAR data for the tops of trees and vegetation? Why would they have that?
Oregon and Washington. I'm not certain about Oklahoma, but they have a very active GIS effort at the statewide level there so I would not be surprised. Why? I do not know, particularly. It's data collected at least part with public money, why shouldn't it be made available??ÿ
Maybe just a slight oversight but just seems to me that 5 or 10?ÿyear old Lidar data might be fine for fixed obstacles, but trees have this tendency to grow and the difference in the height of the crowns over time could be substantial. Some?ÿspecies like tamaracks and cottonwoods?ÿcan easily grow 3-5' a year around here. I often visit locations that were cleared ten years ago and there are?ÿnow 15-20' tall trees growing and it requires clearing again.
Just because I'm paranoid, doesn't mean they aren't out to get me.
Check your field software. You may have Remote Height. Take a shot beside the tree, tilt scope to top of tree and instrument will read out height.?ÿ?ÿ
Maybe just a slight oversight but just seems to me that 5 or 10?ÿyear old Lidar data might be fine for fixed obstacles, but trees have this tendency to grow and the difference in the height of the crowns over time could be substantial. Some?ÿspecies like tamaracks and cottonwoods?ÿcan easily grow 3-5' a year around here. I often visit locations that were cleared ten years ago and there are?ÿnow 15-20' tall trees growing and it requires clearing again.
Very true. But I gather that we are talking about mature trees here, so maybe not so much. But a consideration to be sure.?ÿ
One of the first time I heard of LIDAR being put to a practical application it was the Bonneville Power Adminstration using it to monitor the vegetation height under their transmission lines. It seems that the lines will sag under load and heat, which sag can be modelled and predicted. BPA has a fleet of LIDAR mapping planes which monitors the vegetation height under the lines. When the predicted sag equals the vegetation height line clearing crews are dispatched.?ÿ ?ÿ?ÿ
I've only done one obstruction survey, and it was at a small lightly-used general aviation airport.?ÿ Part of the task was to determine the status of a dense stand of trees about 1/4 mile south of the runway.?ÿ I established elevations at the runway aprons, calculated the approach surface in the office, then returned to the site.?ÿ I set up on the apron centerline with the instrument axis on the 20:1 approach surface.?ÿ I turned the horizontal angle to get on the runway centerline, then turned the calculated zenith angle (the approach angle) and noted that no objects were below the line of sight, i.e. none penetrated the approach surface.?ÿ I placed a note on the map?ÿ stating same, and that seemed to make everybody happy.
?ÿI established elevations at the runway aprons, calculated the approach surface in the office, then returned to the site.?ÿ I set up on the apron centerline with the instrument axis on the 20:1 approach surface.
This only works when the airport is small and quiet. Some years ago I had to do something similar at the main airport on Malta (Luqa). Trying to thread the survey in between continual activity of tourist flights was rather interesting and sat in an airport security vehicle only a short distance off the threshold as things landed was quite an experience.
The point to get across in this discussion is thatoutside the airport boundary the heights do not have to be exactly right - they need to be a good indication of the obstruction top elevation relative to the airport datum. The ground height isn't really relevant, so I don't know why it was asked for (see the original post on this thread).
On Norman's comment on power lines, the UK have routinely LIDAR flown their lines for many years, looking at tree growth. Again, it doesn't have to be accurate. The sag as a major line goes onto full load is substantial. I've carried out observations as part of line load verification and a 275kV line can drop several feet in a few seconds.
The engineer has made the decision that lidar isn't an option for this project.?ÿ So the plan of action is to locate the tops of the trees that I can see on the outside border of the project, and 'assume' the middle has similar heights for planning and rough quantity purposes.?ÿ The project has gone from a select cut to a clear cut, so individual trees are not going to as big of a deal anymore.?ÿ Once the clearcut is done, then I will go in and do an accurate survey of the remaining trees to verify enough was removed.
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I still want to try the drone thing as I think it will be useful in the future.?ÿ The FODAR looks promising but I was having a bit of trouble finding info on that in the VERY short time I looked (I will do more digging on that)?ÿ I figure if they can do stockpiles, then they should be able to do treetops too, so I will research it some and may try it if I have a chance - however the fall schedule may be too full to mess around with.?ÿ If I get any results, I will post up here for sure.
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Thanks for the ideas and thinking points.
I'm no LIDAR expert, but I thought the whole point was to get data UNDER the tree cover....?ÿ Is there a dataset for the actual tree canopy heights??ÿ Our local county data is all "allegedly" ground data.?ÿ?ÿ
Andy-
Thats usually the idea if we are doing DTM work but sometimes its very useful to have above ground data like tree tops or powerlines.?ÿ Just depends on what the final goal is at the end of the day. 90% of the time its just good DTM data.?ÿ
Andy,
Most aerial LiDAR provides multiple returns per pulse as a result of the size of the beam at ground level. ?ÿGenerally speaking the last return is the ground.?ÿ The processed data can be classified by returns.?ÿ As Jake said, most data we as surveyors see is the ground classified data for use in DTMs.?ÿ
sat in an airport security vehicle only a short distance off the threshold as things landed was quite an experience.
Many years ago I was on a crew picking up topo on the end of the main runway at Vandenberg AFB while a KC-135 was doing touch-and-go practice.?ÿ The tower sterilized the first 1,000 feet of the runway, but I felt like I could have poked the belly of that plane with the 25-foot rod I was carrying.?ÿ He must have come over a half dozen times before we got done.?ÿ When you're that close you really get an appreciation for the amount of yawing a big plane can do as it lines up on the runway, and each time you hope that big beast manages to stay in the air.