As I recall, rule of thumb is 1ppm for 11 miles from point or line of coincidence (origin point for Oblique Stereographic, meridian for Transverse Mercator or latitude of origin for Lambert). And 1 ppm for every 20 feet of change in vertical. Just FYI
John Putnam, post: 443219, member: 1188 wrote: Melita,
I think Linebender is referring to low distortion projections. They basically involve utilizing a standard projection (Transverse Mercator or such) centered and sized over the target area to reduce the grid scale factor and then manipulating the ellipsoid to minimize the sea level correction factor. While you can achieve the later by modifying the ellipsoid a better practice is to determine a projection scale factor at the origin of the projection that best fits the local terrain. This projection scale factor is a common component of any projection. That is how we did it in Oregon and if I'm not mistaken, that is how the WicCRS is computed.
I never manipluate the ellipsoid. That is adjusting the datum and does cause problems. All manipulation we do is in the projection.
thebionicman, post: 443260, member: 8136 wrote: I never manipluate the ellipsoid. That is adjusting the datum and does cause problems. All manipulation we do is in the projection.
Agreed 100%
NEVER screw with the Ellipsoid Definition. When you do, you not only lose your "link" to NAD83 (or NAD27, or ITRF whatever), but you create problems down the line, that are much worse than the ones you are trying to mitigate in the first place. I'm not saying that there isn't some value in Ellipsoid manipulation for some projects (science projects), but those kind of things should NEVER leave your office.
Mike Potterfield once said (paraphrased);
"Using a Custom Ellipsoid to solve a grid to ground issue, is like using an H-Bomb to kill flies."
Loyal
I think Minnesota used scaled ellipsoids for their regional systems but it made the work much harder for users.
I know your contract won't allow it, but the easy solution every time is not to use any projection. State plane,UTM, ect.. were invented when surveyors had to do calculations by hand. They were a great and neccisiary time saver then. Now they just jet in the way and over complecated things as this thread illustrates. We all have the software to use real lat longs, ground distances, and geodetic bearings. Why are we still messing around trying to make a system designed to cut corners more accurate. The corners are gone.
aliquot, post: 443278, member: 2486 wrote: I know your contract won't allow it, but the easy solution every time is not to use any projection. State plane,UTM, ect.. were invented when surveyors had to do calculations by hand. They were a great and neccisiary time saver then. Now they just jet in the way and over complecated things as this thread illustrates. We all have the software to use real lat longs, ground distances, and geodetic bearings. Why are we still messing around trying to make a system designed to cut corners more accurate. The corners are gone.
Someday we will be there, not just yet however.
MightyMoe, post: 443282, member: 700 wrote: Someday we will be there, not just yet however.
You're right Mighty!
Considering the number of folks that still believe that the Earth is flat, and even more bizarre, the number of Surveyors who don't yet grasp the implications of a spherical Earth (not to mention an Ellipsoidal Earth)...IT AIN'T going to happen anytime soon.
🙁
Loyal
Next survey convention, maybe I'll rattle a can... To take ip donations for the flat earth society!:)
Not that I'm a flat earther...m hmm but to see how much money it'll raise!
I keep all my data tied pretty close to OPUS solutions. Everything at the basic level is in geodetic coordinates. I have software that will handle this with ease (TBC). Changing projections and going back and forth between as many as I wish is as easy as changing clothes, at least in my space. If you publish something and the base data is not included then there may some issues and decisions to make. Just provide the proper projection/meta data with your product and "most" that know what they are doing should be able to use it properly.
For those that don't know how to use the data I'm not sure even putting it in some standard SPC or such will stop stupid stuff from happening.
If the goal is low distortion SPC is probably not the way to go. A good set of SPC coordinates, if a third elevation coordinate is given also would be able to be translated almost as well as solid geodetic coordinates if they are correct. One problem is that once the data has been translated a few times you could have anything if some errors were made.
This is like groundhog day for these boards! Same groundhogs, same opinions and attitudes. SO, back in my hole.
Oh good, it's what I thought it was. I was worried about the use of "geographic user defined system" equaling user-defined geographic coordinate system AKA a custom datum and/or ellipsoid.
I'm very familiar with the LDPs. Michael Dennis periodically tells me how Esri is doing geodetic things incorrectly or that I'm being imprecise when I do presentations.
LDPs as designed the way that Oregon, Iowa, Indiana, Wisconsin, etc are done are easily supported in Esri software. I love 'em.
I've heard rumors that Minnesota plans to re-do their county systems as LDPs in a year or so.
Melita
