low distortion projection

Loyal is awesome, I've gained a lot of insight from him. I hope to write an article someday about this. I've been using them for about five years now. The way I've been testing them is to determine the geographic extents of my projection area, and view the extreme elevation spreads over the intended projection area on a quad map. I then get a geographic position on some grid pattern covering the area (perhaps 8 points over a city sized area) and determine the combined scale factor of each of those 8 points using the extreme high and low elevations. I then look to see if those csf's are within my tolerance if I were to ignore them (which is kind of the whole point). So perhaps in a mile I may accumulate a systematic error of .01-.02 foot. I can accept that, particularly when my conventional traverse is typically anchored to geodetic positions determined by GPS on a much closer interval than one mile (so it is like a reset).

Shawn has provided some good advice. I was on the technical development team when ODOT developed the Oregon Coordinate Reference System. Lots of good info in the OCRS Handbook. Appendix B is the results of testing we did.

Basically I think we nailed every projection, the test results verified that, BUT the methodology we used on the input side almost guaranteed the field results would check out.

SHG

thank you both. i am currently checking out my computations. shelby, that particular pdf is the most complete resource i have found yet. i will keep you informed

Most software/data collectors will output the ground distance and the grid distance. So with a LDP going on its easy to compare the LDP grid distance to the actual ground distance. If your LDP is working good and you are not trying to stretch the limits to far the two distances shouldn't vary too much.

I've been promoting LDP's for Oklahoma to anyone who will listen. Since the elevation does not vary as much as it does in Oregon I think it would be much easier to set up. Probably thirteen rectangular Lambert zones would cover the state nicely.

The 13th would be the "Panhandle Zone".

I recently utilized the OCRS handbook to create an LDP and would like to commend the authors and contributors of said handbook for creating an excellent resource.

The intent of the LDP was to create a projection in ARCGIS from state plane (utilizing the underlying NAD83 geographic coordinates) to a local ground based engineering grid. The Rectified Skew Orthomorphic Center projection was used since the local grid's North azimuth was skewed significantly from its point of origin's geodetic North. In order to compensate for said skewed azimuth, the projection's Azimuth parameter was set to match the skew from geodetic north, and the X_Y_Plane_Rotation was set to 0. Slight adjustments were made to azimuth parameters and to the scale factor to best match accepted local grid values.
When tested against said local values on 20 points, the largest accepted mean error (Northing) was .005' with a standard deviation of .003'. The subject test area is close to sea level, has very little change in topography and the geographical extent is less than 3.5 miles.

:good:

The Minnesota DOT developed County Coordinate Systems which are low distortion projections.

See:

http://www.dot.state.mn.us/surveying/ToolsTech/mapproj.html

There is much good information on this web page which could be a guide to creating other LDPs.

I've heard rumors that there's also one that covers the Denver metro area, although as far as I know, it still hasn't been published.

We've created our own systems for the Woodland Park and Colorado City areas in Colorado, and they seem to be working very well. And we've included all necessary metadata on all our recorded data. But they haven't been officially adopted by any governmental agency. We're doing construction projects that use them, however.

And some things get funky... For example, in Pueblo, the county wants to use Colo SP South (NAD83), because of their GIS system. However, most plats are on an assumed ground system, and may even go back to the "one-eyed goat and a tape" system. And the town of Pueblo West was platted on Colo SP South (NAD27). Conversions are possible in some cases, but they include details that can cause issues, especially when lawyers get involved. And the worst are the older sections, where plats done 100+ years ago are not accurate, and the monuments have been destroyed.

But this is one of the reasons why I think Surveyors won't disappear soon... Things like this can't be sorted out without professional knowledge. I reject the idea of some (which I've seen in some of the recent industry mags) that Surveyors are not professionals, or are being made obsolete by GPS.

> But this is one of the reasons why I think Surveyors won't disappear soon... Things like this can't be sorted out without professional knowledge. I reject the idea of some (which I've seen in some of the recent industry mags) that Surveyors are not professionals, or are being made obsolete by GPS.

:good:
I believe this is one of the reasons that GIS departments need to have a surveyor on staff to handle the spatial component of their data (especially to qualify it). For this to happen, surveyors need to ensure proficiency not only in the GIS software utilized but also in pertinent geodetic applications.
I am not implying by any means that surveyors should run the GIS departments unless they have the requisite training and experience.

Sorry, but knowledge of geodesy and how to use it is technical not professional. That's why the NGS state advisors don't need a license yet they know as much or more about datums and how to work with them as anyone. Datums are black and white even though technical mistakes can be made determining them and using them. Boundaries are grey. Professional judgement is needed for the grey. And don't get me wrong, the technical expertise may be as valuable or more so than the professional expertise at times.

Yes, there is a Denver LDP and no, it is not published yet. Our goal is to someday replace the City's GIS reliance on state plane coordinates with our ground based LDP. Working at the City has taught me patience. I now admire the comparatively blazing speed of a glacier.

> Yes, there is a Denver LDP and no, it is not published yet...
Mike Berry maintained a Central Oregon Zone many years before the Oregon LDP system was developed. I believe that his zone was simply incorporated into the overall system.

I would consider topographic, construction, and control surveying to be black and white (in most cases), yet these functions are included in many state statutes' definition of land surveying.
I simply believe that having a knowledgeable licensed land surveyor within a GIS staff to qualify spatial data would make said data that much more valuable.
I don't question the professionalism necessary for boundary surveying. The first state I was licensed in defined the role of the professional land surveyor as boundary related only.

> Sorry, but knowledge of geodesy and how to use it is technical not professional. That's why the NGS state advisors don't need a license yet they know as much or more about datums and how to work with them as anyone. Datums are black and white even though technical mistakes can be made determining them and using them. Boundaries are grey. Professional judgement is needed for the grey. And don't get me wrong, the technical expertise may be as valuable or more so than the professional expertise at times."""

i'm with you on the gray part of the professional judgment for boundary determination, etc. However, with respect to ldp development, i believe it is no less professional vs. technical. a good ldp takes forethought, planning, manipulation and testing. they are easy in some respects and more difficult in others. projections, datums and reference frames are a necessary part of a licensed surveyor's knowledge, and are reflected in the exams and experience. something to think about, linebender. please reply, i look forward to more dialog...

There are a couple of distinct things to look at here...

Yes, one is the technical knowledge of how projections, etc., work. But the other is how the Law fits into this. One of the requirements of being a PLS is that you are half-lawyer, half-surveyor. And of course, these days, you also have to be half-GIS (OK, I know that makes three halves, but bear with me). This blend of knowledge can't be defined as anything less than professional. It's a very complex blend of very disparate categories of knowledge, and something not everyone can pull off. Maybe our licensing standards need to be revised, to acknowledge the new technology, but we still need to know our English Common Law stuff, all the way to our cutting-edge GIS technology stuff. And that's leaving out the basics, such as knowing how to use the latest CAD technology, how to use LDPs or standard projections (State Plane, UTM, etc.) in general, and more, so we can make things happen in this new age.

It's easy to be a half-assed surveyor, at least up until you get fired for failure to keep up, or for some critical mistake. One of the jokes we used to tell back when I started (in the mid-80's) was "A Surveyor is someone who goes to the bar every night, and complains about what happened all day, then goes to work the next day, and talks about what happened at the bar last night..." These days, you have to be on top of your game, or go down. And it's a very different game than we saw prior to even 2008, if not later. We now have even more tech coming into play, such as scanners that can shoot 1Million points/sec or more, GIS, software like Civil 3D and Carlson Civil, "imagineering" software like Autodesk Infrastructure, "3D video game"-like interfaces like NavisWorks, and more.

This industry is no longer for the people who want to just go out and pound blue-tops, then spend the night in the bar...