Just finished a photo control job where the end client (a city) insisted that all ground control points for the imagery MUST have a rebar set. At first they wanted 6 foot long rebar, talked them down to 18" length rebar. But, what that meant was no painted targets (which is what I typically do if no ID's are present), all had to be vinyl targets (we call them portables) that are of course very susceptible to people removing them before flight. And that also meant no easy photo ID's like stop bars, etc. I was able to use two sidewalk corners as ID's because I could put a rebar at the corner.?ÿ
I think their reasoning is that they want to reuse them on future projects, their own personnel went out and targeted some previous control. But, I think a good ID like a sidewalk corner is as permanent if not more so than a rebar at a vinyl target. My problem is that a lot of times the public will see a target and think it is for a highway, or a development, and they will rip them up. I set as few portable targets as possible. My preferences are ID, painted target, and last resort portable target.?ÿ
I also see a lot of cases where surveyors paint a target when there are plenty of good ID's nearby. Why??ÿ
I've painted because, the vinyl get ripped up, or ate by livestock. Corners of walks, or breaks between concrete and asphalt are great. Only problem is I have had a corner of a walk that I shot covered by grass two weeks later.?ÿ Corners of stop bars are also good - unless there is a car there at the perfect time...
I looked at a big photo control job recently where we would have been using vinyl targets - I was going to set rebars precisely because they could easily get removed, whether by people, livestock, strong winds, etc. Where this was using photo identifiable features probably wouldn't have been possible.
Better to have more GCPs and tie points than too few...
That is two funny!?ÿ?ÿDid they have a city surveyor or just a city engineer??ÿ Sounds like a typical engineers decision.?ÿ As absurd as the architectural canned spec, "Construction surveyor will set 4" brass disc to control construction from"?ÿ with no comprehension that the construction surveyor needs to follow in the design/topographical/boundary surveyors footsteps and control for construction.?ÿ I set a lot of aerial control in my day that I would continue to use for site control after the aerial.?ÿ First it had to meet the aerial companies criteria but second I looked for easily recognized spots on durable long lasting hard surfaces (scribed "+" or drilled lead and tack in concrete,?ÿ preferable found) if available that would be easy and safe to occupy.?ÿ It was always beneficial if some of the points landed on controlling monuments such as center-line intersections and such.?ÿ The cost and logistics of making sure the vinyl targets are in place at time of flight is not realistic in most instances, especially in an urban environment.?ÿ Jp
That is two funny!?ÿ?ÿDid they have a city surveyor or just a city engineer??ÿ Sounds like a typical engineers decision.?ÿ As absurd as the architectural canned spec, "Construction surveyor will set 4" brass disc to control construction from"?ÿ with no comprehension that the construction surveyor needs to follow in the design/topographical/boundary surveyors footsteps and control for construction.?ÿ I set a lot of aerial control in my day that I would continue to use for site control after the aerial.?ÿ First it had to meet the aerial companies criteria but second I looked for easily recognized spots on durable long lasting hard surfaces (scribed "+" or drilled lead and tack in concrete,?ÿ preferable found) if available that would be easy and safe to occupy.?ÿ It was always beneficial if some of the points landed on controlling monuments such as center-line intersections and such.?ÿ The cost and logistics of making sure the vinyl targets are in place at time of flight is not realistic in most instances, especially in an urban environment.?ÿ Jp
Tell me about it.?ÿ I do this nearly everyday.
And then they want you to remove the targets after the flight...1300 miles away from my office. I get that request, until I tell them the cost to revisit each location.
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I also get requests to just panel existing NSRS monuments. That usually winds up costing MORE than just getting IDs because a lot of times they cannot be easily targeted, or the mon is in someone's yard, hard to find, etc
