Does anyone have a rule of thumb that they go by when surveying around a building with a GPS concerning how close you will get to the building and still take a shot?
When the TOP of the building is BELOW 10-15 degrees(vertical), relative to the phase center of the?ÿGNSS antenna?
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Not so much of SOP but when I am close those sketchy areas, I keep an eye out on my Coordinates, if it jumps through big numbers even if the DOP are good, I do not take the shot.?ÿ
Javad LS can take some really crazy shots. I took some under the overhang, metal bldg, roof was some 20' up. It was not near a boundary, and i was not in a big hurry. Still, I spent some 50" on them average. They take longer, in harsh environments, and their error elipse tends to be larger.
Building had a short side, by 0.20' (inverse versus tape)
O well. I had to try it.
No matter the brand, I usually suggest 10' offsets. Higher confidence, and goes faster.
Or, extend your gps pole, and get the eave corner.
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Buildings present two problems. They block the sky, and therefore block satellites. This is predictable, but depending on the severity can cause your DOPs to rise to unacceptable levels. They also create a high multipath environment, which raises the noise level and can result in poor or incorrect results. I can't come up with a single rule of thumb that accounts for this, it just depends on the conditions and what you're trying to accomplish. Sometimes GPS isn't the right tool for the job.
What I have been doing for mapping buildings with GPS is to stay as far away from the building as I need to maintain acceptable tolerances and then shoot the offset distance with a handheld laser distance meter.?ÿ I usually store two per wall and label my points (distance)os(n,s,e,w) so I remember which way to offset my line in cad.?ÿ I have compared this method with direct readings from the robotic and have had good results.?ÿ The laser handheld distance meter is much smoother than wrangling a tape through landscape and weeds, etc.?ÿ As far as getting a good lock in proximity to a building, it seems to depend on the height of the building, satellite configuration, and other surrounding obstacles.?ÿ For single story buildings it seems like 6' has been the minimum offset that has worked, whereas a taller building has pushed me back to 15' or even 25' in a few cases.?ÿ?ÿ
I commissioned an as-built for a group of warehouses last year, mostly tall over 10 meters, and close together... about 3 hectares.
The surveyor had it done rapidly, just a couple days. That surprised me, I thought it would be more than a week so I asked, he said it was all GPS, one man rover with a local base station.
He used a laser rage-finder, but only if he could not use his pocket tape. He said he mapped everything, had lunch and then did it again. He went back the next day to resolve a couple blunders. I did not get into the details but I am highly impressed.
I will go ask about what brands and models of hardware he uses... we typically have older hardware here in Baja. Cost.?ÿ
