A client has a modified state plane NAD83 system (scale factor around a project point) and wanted to set-up an AutoCAD drawing to use it and be able to query in other drawings, photos, ect. I'd never had much success setting up autocad with anything other than one of the predefined systems. I told them as much but they were too frustrated not to try.
We set down with their computer system (they have the latest autocad-2014) and started to try and make it work. The computer tech had gone through all the menus looking for something that would do what we wanted and was showing me what he had found when he brought up a slide with an option to put in a transverse Mercator projection.
Hello!
We figured out all the parameters and bingo the projection meshed perfectly with the modified state plane. In fact we were able to do another modified state plane system set up for data and drawings that were collected and drawn since back in the 70's that are based on NAD27 (also modified around a project point). He was able to query in that info into the NAD83 drawing along with quads, orthos, and other info and it all came in seamlessly.
I must say the later version of autocad handles this really well.
bingo
There is no such thing ....
as a modified state plane system. It is State Plane or it isn't!
There is no such thing ....
modified is a frightening word with respect to projections. duck and cover
amen
amen
There is no such thing ....
Well it's a state plane system that's been projected to the surface of the earth so it can be used in the real world. All the bearings are state plane grid and distances are state plane adjusted by a scale factor. So I guess I'll continue to call it modified state plane like I have for the last thirty five years. The cool thing is that in a few minutes I can set-up auto cad to use a modified system and the coordinates will match the lat longs. For modified 83 or 27. I know these systems scare many surveyors but not me. Never have never will; never had a problem with them. Easy to use in the 1970's and even easier to use today
There is no such thing ....
"Well it's a state plane system that's been projected to the surface of the earth so it can be used in the real world."
That's called "ground"!
It's a local coordinate system derived from state plane. It's a wonderful thing if it meets your needs, all the conversion metadata is supplied, and you subtract off the millions and hundred thousands in the conversion so that no one will mistake it for real SPC.
Can't subtract the millions. Won't work with the scale factor.
Was taught many years ago, pre-computer age, to average the computed grid factors at the edges of the project. The single combined factor was then used for all computations of project grid distances.
This appears to be a modern version of the same principle. As long as the scope of project and elevation difference do not produce error in excess of the survey accuracy required by the project this has long been an acceptable procedure.
Originally, back in the early 1970's, a control network was run using NAD27 state plane. This was done by occupying local NGS NAD27 triangulation monuments, holding the coordinates on one of them, surveying with ground distances and checking into the other triangulation monuments after "adjusting" the coordinates for them to reflect ground values. This worked quite well, you got a "modified" state plane system that was "close" to state plane, you could plot on quad sheets and survey on the ground without changing each distance as you went.
This was very important because there was a considerable amount of construction tied to the control. No one wanted state plane distances to be used. Fast forward to the 1990's and a neighbor developed a control system in NAD83 with state plane coordinates scaled up around a project scale point on site. Since then the two properties were merged and it has become important that the old 27 values and the new 83 values be merged together holding the NAD83 numbers.
Of course, the two systems are quite different and holding two common points and rotating and shifting has been the tedious process to match the systems. By allowing autocad to project the new NAD83 modified state plane system in a drawing it has allowed other older data (not surveyed) to be imported into the new drawings. That's important since the data transfer is time intensive and this helps greatly. Not only the 83 system, but also the 27 system can be sampled into the main drawing. Since there is no real way to translate between 27 and 83 you still need to be very careful just what kind of data gets sampled into the new drawing but it is working better than anyone thought it could.:-D
