Aerial Panels Question

I would say that his method is sufficient if he has nice clean lines at pert near 90. Maybe a quick diagonal check with a pocket tape and it's more than sufficient. I would even think that 12'X12' targets for high altitude photo's wouldn't have to be as critical.

LOL I guess I should have mentioned that the last time I did them was also in the late 90's.

SC Crew Chief, post: 324913, member: 8876 wrote: How accurate do chevrons need to be laid out?

Imagine that you were sighting your photo mark from 2000' above with the telescope of your total station. That is a close simulation of what the photogrammetrist is doing. How close to 90° does the mark have to be to get a good sight on the apex?

Cleaning up the edges with an inch or two of black helps them jump off the photo, especially on concrete or old asphalt. I've had photogrammetrists comment favorably when I do that.

Have you asked the photogrammetrist?

I worked for a photogrammetry company for 20 some years before retirement. Our photogrammetrists loved it when the targets were done like Norman just described. It makes a measurable difference in their residuals. I actually used masking tape when doing the target painting. Nice crisp edges and black on the outside when on concrete or old asphalt.

24 inch panels means a low (2000 feet or so) flight. They can really zoom in on a target from that height. They always wanted the reference point to be at the tip of the chevron. For higher flights (say over 4000 feet) we commonly used three panels arranged as a propeller and left the center open with the actual point being in the center marked w a 60 penny nail w punch, on dirt and a PK on asphalt. Makes a nice target.

I work in the aerial business, for less than $100, I had an aluminum stencil made, makes quick work and nice clean edges. The way my stencil is designed, set nail first, then paint, target is perfectly centered on center monument AND leaves a square in the middle non painted.

I rarely set two legged targets unless the on the ground circumstances do not permit something else. Our order of preference is a "+" or "Y", followed by a "T" and last resort is the "L". It is much easier to sight the CENTER of a symmetrical target IE the "+" or the "Y" than a non symmetrical target IE the "T" or "L". Even on the "L" style, I rarely use the corner as the point, always the center of the intersecting legs, there are rare exceptions such as using the corner of a large concrete block, but that is about 1% of the time. Whatever you do, make sure it is very clear to the photogrammerty folks where the surveyed point is located. Place target on a flat level surface is possible, certainly flat if not level (constant slope) in an area that will NOT be shadowed at time of acquisition.

SHG

When I was with the highway dept. (this was '93 or so..) we prepared control for a low altitude route flight. Although our target horizontal coords were generated by the software and forwarded to the aerial department for their work, our vertical control actually wound up being typed by hand. We did use digital levels, but it was necessary to hand type the list of target elevations to "clean" the copy up from the intermediate turns.

Aerial called me one day and had a target they couldn't focus on. She was pretty sure I had busted something. I checked it out and sure enough, I had transposed some numbers on a target's elevation. The elevation they had determined was within about 20mm of what it actually was.

I've had a healthy respect for photogrammetry since then. And that was twenty something years ago.

paden cash, post: 325032, member: 20 wrote: When I was with the highway dept. (this was '93 or so..) we prepared control for a low altitude route flight. Although our target horizontal coords were generated by the software and forwarded to the aerial department for their work, our vertical control actually wound up being typed by hand. We did use digital levels, but it was necessary to hand type the list of target elevations to "clean" the copy up from the intermediate turns.

Aerial called me one day and had a target they couldn't focus on. She was pretty sure I had busted something. I checked it out and sure enough, I had transposed some numbers on a target's elevation. The elevation they had determined was within about 20mm of what it actually was.

I've had a healthy respect for photogrammetry since then. And that was twenty something years ago.

Me too, have run into it multiple times over the years. One of the first I recall was about 1982-83, we targeted a section corner pipe that was a bit above ground level and forgot to subtract the monument height from the cap elevation, the photogrammerty guys called and said you might want to check that height, LOL. I don't remember the exact details, but it seems they nailed it within 0.25 feet from a fairly high flight and of course much more analog plotters than now days. Of course I see the shoe on the other foot quite often too, when the AT goes sideways and the ground control gets blamed, at least initially, but usually that is caused by a bad tie point or reading of the control, having the ground control and the AT (and AGPS control usually also), sure adds a bunch of checks, if everyone works together to get an acceptable AT solution it produces results that are amazingly good on the ground (within the confines of what you can see, still can't penetrate vegetation very well). Now with LiDAR, we can create bare earth models that are generally pretty good, technology sure has changed in the last 30 years or so!

SHG

Shelby-

The stuff you guys (3Di) did for the City of Yreka last year; We went back and set vertical reference marks on head walls n' such. We were hitting you guys within hundredths on most of them when I went back to the provided vector data for a reality check on my levels.
:good:

Thanks! Got to like that kind of a check! I didn't even have any involvement on that one either, LOL.

Obviously to get hundredths the ground control can't be slopped in, it starts to get important to have crisp target edges, the visible target centered on the surveyed point, the surveyed point corrected to the visible surface if not coincident, etc. Pretty phenomenal what you can do these days with digital cameras and a soft copy workstation, a much much better product can be produced than in the old film and analog plotter days.

SHG

Thanks! Got to like that kind of a check! I didn't even have any involvement on that one either, LOL.

Obviously to get hundredths the ground control can't be slopped in, it starts to get important to have crisp target edges, the visible target centered on the surveyed point, the surveyed point corrected to the visible surface if not coincident, etc. Pretty phenomenal what you can do these days with digital cameras and a soft copy workstation, a much much better product can be produced than in the old film and analog plotter days.

SHG

Next time you talk to Brett or Ron; tell them they knocked it out of the park.
Thanks!

Thanks, will pass this on to them! Nice to hear when the end result is nice and tight...

SHG

http://www.berntsen.com/Surveying/Flagging-Targets/Aerial-Targets-Aerial-Paneling

cptdent, post: 328003, member: 527 wrote: http://www.berntsen.com/Surveying/Flagging-Targets/Aerial-Targets-Aerial-Paneling

As a word of caution, the vinyl panels are prone to theft. I've done quite a bit of aerial mapping. In Louisiana, people see it as a free tarp for the taking. Lost a few on large scale project for Orleans Parish. Fishermen (both bank & boat) and others helped themselves. Luckily we set or adopted auxiliary targets on hard surfaces or other identifiable features.
The rolls are better and less subject to theft or damage. One survey contractor that I know would buy highway striping rolls to compliment.
They had an adhesive backing too.

My favorite was painting 8 balls. Templates were made out of plywood.