Hello,?ÿ
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I am working on a project that has multiple maps, and I've decided which map to hold as my basis of bearings. That being said, what is the proper procedure for rotating maps to that map? I have an idea that there has to be a common line to determine a delta of rotation between the maps, but say I start adding other maps that relate to that 2nd map, do I have to rotate to match that 2nd map, then rotate to match the main basis of bearings map I selected? Any help on your thought processes, procedures, etc. would help immensely.
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Regards,
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Scrappy
Don't rotate to any of them. Use a standard projection such as a State Coordinate System and hold that, otherwise you're rotating, rotating, rotating with each iteration and before you know you've spun 360 degrees.
Seriously, if you're planning working in the same area over and over, it's best to scrap the notion of continually rotating to existing monument record bearings. That becomes a chasing your tail operation. I don't recommend it. If you wish to make a reference to original bearings use a record notation.?ÿ
@mightymoe Thanks for the response Moe. Could you explain a little more on how I would hold the SPC?
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Scrappy
@scrappysurvayir If you are surveying using GPS in real world coordinates, why would you want to rotate those coordinates to something other than. Compute your plat or whatever using record bearings/distances and using the best fitting control you??ve recovered, translate and rotate your comp points to the best fitting monuments you??ve recovered and tied. Get familiar with the scale factor in your area for the standard projection you??re using to scale the record ground distances to grid once you??ve worked out how your tape measures up against the surveyors you??re retracing. You can then scale those ground distance to grid to tighten things up. You want a consistent repeatable source of bearings across the full scope of your project. What you were proposing borders on madness!
We would always survey from known SPC (State Plane) control monuments to get onto State Plane bearings. With the advent of GPS it's a simpler process, pick your wanted system let the GPS receivers, programs and Data Collectors project the Geographic coordinates onto your survey. It's always best to "be on" the coordinates by using CORS or OPUS to get there.
The RTK points starting from autonomous coordinates without corrections to CORS will give you correct SPC bearings. The coordinates will be off a slight amount usually something less than 10 feet in a worldwide sense.?ÿ
If you don't have GPS or access to it there are probably any number of resources with SPC coordinates nearby, usually DOT maps or Subdivision maps, or any number of nearby projects may have what you need. If that's not available to you there's the tried and true method of astronomical observations. I personally haven't done one since the mid 2000's, but it was a common task not too long ago.?ÿ
On the other hand, I more often do not use SPC.
For multiple maps and utilizing CAD, I'd do this:
Draw each map as given in a separate DWG file;
Xref each drawing into a "Master" drawing.
Rotate all xref's to your basis of bearing drawing using common lines.
Alternatively, use Insert as a block instead of xref and perform the rotation.
I agree with Moe, I rarely use an old basis of bearings and then only when I have a very good reason such as their is no practical way to get some GNSS ties (miles of deep 200 foot high coastal forest).
Multiple maps rarely fit together perfectly and they rarely fit my survey so the means I use to match them via translate and rotate is somewhat arbitrary based on my needs. Usually it??s just a convenient way to have search coordinates. Occasionally there is a set of old maps done by the same company around the same time so they will tend to fit together perfectly since they copied their own numbers.
I usually use Geodetic north at a central physical mark and scale to ground with sufficient ties to Geodetic control on the map along with all the required metadata so that another surveyor can calculate my map in state plane grid if they want to.
I think it depends on the nature of the survey.?ÿ If I'm working with large parcels, then I agree that SPC or similar is the way to go.?ÿ But if I'm doing an urban lot survey, I'll usually pick a monumented line on or near my parcel and rotate everything else to that, using whatever ties to the other maps I have available.
Even though I often use RTK on a lot survey to make ties in adjacent blocks so I don't have to traverse there, I'll stick with a local bearing reference for the resulting map.
I have rotated to match highway deeds numerous times.?ÿ Especially the modern ones with 27 ins and outs in a half mile.
