Land Survey

[space-time]

I saw some survey crews on a the road I go to work lately, in fact over the past 2 weeks or so. They have those yellow tripods and measure stuff and so on. Now they are painting some white crosses on the middle of the road, about 4-5 per miles. I stopped at one of these as the guy was literarily watching the paint dry. I asked him what this is for, he said for an aerial survey. This is a back road through the woods, no traffic, and I guess these would be hard to see in the summer as the road is covered by trees, although they seem to paint these white crosses in some spots with fewer trees such as driveways or other areas with more open space.

Any idea why they can do with these? also why do they still do these surveys with those old fashioned tripods and optical instruments, can't they use GPS? I think the encrypted GPS signal is much more accurate than the unencrypted signal used in our GPS devices, but I would imagine these companies could get some compromise from government to allow them to use the encrypted signal for very accurate surveys.

[M>B>]

I have seen this several times. It us most likely for planing a road alinement or some such. It might be years before they do it. Road planing is like a master plan and the look far into the future.

[Paul F.]

Shooting buddy of mine was a surveyor for years, civilian, not government.
He has a pretty low opinion of state "survey crews". Rightly, or wrongly, I don't know... he has gone on a rant about 'em before "bastards take 5 guys three days to survey something that I've done in 6 hours...".

From what little I know, modern civilian surveying GPS is accurate to a few centimeters - the intentional encrypted "blurring" of the signal was removed some years ago. It can be selectively turned back on at any moment, either globally, or regionally.
It still takes a GOOD GPS to get that kind of accuracy, however.
That's my understanding, anyway.

This is all second hand, so if there's a surveyor in the house, I bow to their experience.

[mrbigstuff]

in all likelihood, they are reading grades first and foremost. the instrument being used is called a "transit" and is what most road surveyors use to set grades for utilties which is the first thing that must be done (unless there is excavation to do to create a road from virgin terrain.

what you are thinking of a typical land survey, where metes and bounds are surveyed, is probably secondary to their task.

[OWC Jamie]

topography and targets for references

they did an aerial survey for the property next door and the big x on the ground were just coordinate reference markers for the adjoining wetlands and non-wetlands / geographical incidences on the property


multiple x indicate the true map scale from the aerial photo

[OWC Jamie]

space-time wrote:
can't they use GPS? I think the encrypted GPS signal is much more accurate than the unencrypted signal used in our GPS devices, but I would imagine these companies could get some compromise from government to allow them to use the encrypted signal for very accurate surveys.

they do use the accurate signal
great for a point
one still needs to shoot lines

[srf1957]

Or you can have the coast and geodesic survey put a seismometer on your grandmothers place. This happened when I was in high school. Funny thing it looked more like a radio transmitter and The air force started using the local airport as an electronic bombing range. They said it matched a site in the USSR. They took it down as soon as the B-52's quitflying over.

[Speedy]

Laser shooting has revolutionized surveying. It's amazing what modern technology has done for accurate excavating. It's been around for decades but it's still being implemented.

[Mr Downtown]

The white crosses are used during aerial photography. After the photos have been shot, the crosses, which show up in locations whose coördinates are known, allow the photogrammetrist to mosaic together the individual photo frames. Overlapping frames can then be examined using a stereo plotter to derive a 3D view and plot contour lines or a mesh of known elevations.

The typical local government aerial survey, for cadastral mapping with building roofprints, only needs a white cross "target" every couple of miles or so, and these days it's usually accurate enough to locate those with GPS devices. Putting several down in a mile, plus the other fieldwork you see with the devices on tripods, probably indicates work for a road widening or similar project where they have to figure out how much earth will be moved, what the resulting slopes will be, and how high the retaining walls must be.