State Plane Coordinates

I never change my coordinates values...but I'm at elevation 25.:-)

Not certain what you are trying to indicate with “modified” coordinates, but I think I understand the overall post. Here in our area, we had some local jurisdictions implement coordinate systems they thought were SPC. They set-up these systems in the late 80’s and early 90’s when pulling a SPC value from your GPS required some skill. Apparently some of these jurisdictions never acquired these skills and are now stuck with GIS systems reported to be on SPC, but are not. Nothing is more irritating than to have some pimpled face, snot nose GIS “expert” from the city tell you that your plat is “off” and actually wants to know if you “know what you are doing.”

In this world of computer models, do we really need a plane coordinate system? Is there a need to represent a sphere on paper any more? Just wondering.

Isn't elevation only half the equation for the combined scale factor?

If by modify, you mean apply a scale factor as required in an RTK localization, then I don't see your problem. Usualy given the job size (with a proprtional grid containing occupied points from North-to-south,East-to-west) the scale factor wouldn't even be noticeable, I have been on Jobs so large as to require two overlapping localizations, and have run with up to 4th generation control from this "modified" grid.(with good system checks) The scale ratio is usually one to a very large number with up to six places past the decimal. Such as scale factor= 1.0000321 which on that grid is actualy what '1' would equal. I don't see a problem, again maybe I am misunderstanding your use of the word modify. Coordinate geometry, isn't that tricky what with tuff calcs like 'least squares,'(as all things now) being handeled by software. I'm not seeing your complaint with state plane, at least your not in a meets and bounds system.(I presume)

Couldn't have said it better myself. However, my elevation lends itself to near surface distances between grid and ground, so I may have a different view in the mountains, or know more about LDP for that type of work.

Noting: All of these localizations were done in South Florida, 15-20 ft above sea-level.

Gene

I think Loyal is lamenting about the inflation of SPC to ground for various reasons and not about working within a framework for regulatory reasons that were set up by some idiot in an office.

There are a few (easy) ways to get surface distances on a plat and still maintain your original coordinates without molesting them.

The easiest way I've found is to draw everything in grid, and I mean EVERYTHING until you're about ready to write the notes. The last thing you'd do is annotate the map.

Then, scale the linework up only. Annotate the map, and voila, grid bearings, surface distances, and a virgin coordinate file that hasn't been defiled by some cad monkey (or tired RPLS for that matter).

Everything works. I just hate doing it so everything I show is grid.

Loyal, I'm struggling to understand your question. Are you referring to SPC that you did not return; i.e, furnished by a jurisdiction / govt body...a record survey....?

Or you are referring to a coordinate system actually returned by you through whatever field / data collection procedures? Not really understanding the 'modifying' statement either, sorry :-S

Thats a valid point, this is a world of computer models, so why represent a spheroid? Don't you think a cartisan grid makes calculations so much easier? I would much rather travel lat and departure (or polar rectangular) than rely on models. Coordinate geometry is a tool; why not use it as such?

Whether to rescale or otherwise modify the coordinates is a decision that depends on you and your client, the application, etc, and might be debatable.

What is NOT debatable in my mind is that if you do modify the coordinates, you explain clearly how (metadata) and you make them look different enough that they cannot be mistaken for SPC. Usually the easiest way to avoid confusion is subtracting off the millions and hundred thousands of feet.

1) So that 1000.00 feet calculated by coordinates equals 1000.00 feet on the ground.

2) Grid North is still Grid North.

3) State Plane coordinates can be easily calculated from the "modified" coordinates if you have someone requesting the info and they understand the difference.

I agree with Bill93 that all would be better served to subtract out the millions so the two will not be confused. TxDot chose not to do that. You would also have someone somewhere that would subtract 10mil and then multiply by 1.00012 when they should have multiplied by 1.00012 then subtract 10mil.

James

Hillbilly, Butch et. al.

Mea culpa dudes...

I had ASSUMED that “modified State Plane Coordinates” (which I maybe should have defined better) was pretty much a common term everywhere (me bad).

In over 40 years of surveying around the country, I think I have seen ONE (maybe 2) Plats (or Records of Survey) that were actually in “true” State Plane Coordinates. ALL of the hundreds of others, were in some form of “modified State Plane Coordinates.”

Basically two techniques were used:

First (most common around here) is what I call the “GLOBAL modification.” In this form, the State Plane Coordinates are ALL scaled from the ORIGIN (say Y,YYY,YYY.yyy / .999??? and XXX,XXX.xxx / .999???).

The other technique that I see (which I call a LOCAL modification) is where a SINGLE State Plane Coordinate is held as FIXED, and all others are scaled RELATIVE to that point, based on their distance FROM said fixed point!

In BOTH cases, ALL of the Modified State Plane Coordinates are similar to State Plane Coordinates, but are NOT (except in case 2 where ONE of them IS). Some folks truncate a digit or so off the left side of the coordinate values, some do not.

Sorry bout that,
Loyal

Hillbilly, Butch et. al.

So, your seeing someone state "SPC" but then show a ground distance rather than Grid?

We tend to see a lot of grid surveys here. or surveys in a Local datum plane (ground) that scales to the State Plane grid based upon listed parameters.

Our State law:
The Oregon Coordinate System of 1983, north zone, is a Lambert conformal projection of the Geodetic Reference System of 1980, having standard parallels at north latitudes 44 degrees 20 minutes and 46 degrees 00 minutes, along which parallels the scale shall be exact. The origin of coordinates is at the intersection of the meridian 120 degrees 30 minutes west of Greenwich and the parallel 43 degrees 40 minutes north latitude. This origin is given the coordinates: x?2,500,000 meters (8,202,099.74 feet) and y?0 meters (0 feet), where one foot equals 0.3048 meters exactly.

Hillbilly, Butch et. al.

I admit I've done both to get surface distances, usually so they compare to the existing surveys which 70% of the time are on some local basis of bearings and about 30% of the time used a astronomical north (mostly solar). I also rotate it to geodetic north also for comparison purposes. I don't usually map it like that, though. The map will be in state plane grid bearings and distances.

I do all processing in Star*Net. I usually use Static GPS for control but most of the work is done with conventional. Star*Net takes the conventional raw data and processes it in the state plane zone I choose (there is no need to monkey with scale factors on the DC, I just collect it usually starting with an arbitrary coordinate system). At the end I have the project in State Plane. I agree it should be left that way and most often I do leave it that way. If someone wants to know what the ground distance is along some segment of some line in the future then they can calculate it from the metadata on the map.

We also have a Leica Scanstation2 HDS scanner. I could be wrong about this but as far as I know the Leica Cyclone processing softare does not handle projections. I can give it control coordinates to register the scans to but I scale those up to ground also at a local control point (rather than shifting then entire thing from 0,0). These projects usually only cover a few acres so it doesn't make that much difference in the bigger scheme of things.

I agree though, on large projects especially with large elevation differences they should be left either in State Plane or a custom projection. GNSS Solutions has a really nice ground coordinate system generator which will convert your project to a ground system which I can upload to the DC and use for conventional work. I haven't had very much luck getting other software packages to do a custom projection I feel comfortable with. I haven't tried it with Star*Net but I have with SurvNet and couldn't get it to work properly.

Hillbilly, Butch et. al.

Local modification sounds to me like a translation to one point, or a best fit. How have do these to techniuqes compare after scaled? How far can you go with a "local" scale, before it's time to re-grid?

Hillbilly, Butch et. al.

Those are questions for judgment. If you have 2500 feet of relief in your project over a township, for gawd's sakes leave it in SPC.

Hillbilly, Butch et. al.

For example, I have a project which is the east half of the northeast quarter of Section 9 (nominally 80 acres) and the north half of the southwest quarter of Section 10 (nominally 80 acres). These parcels meet at a single point, the quarter corner between Sections 9 & 10 (which has a monument). For purposes of comparing my measurements to the Department's 1959 Record of Survey (solar basis) I scaled and rotated my Survey to ground and geodetic north in a MicroSurvey project. Ultimately this won't be used, it's just for perusing it for now. I put a big note to myself in the drawing in an Mtext explaining to myself what I did so that a year from now when I'm wondering what in the heck I did I can read my note which hopefully will make sense to me.

I've had the same gripe as you do in the past. However, it is part of our standard to modify the state plane coordinates to reflect the approximate elevation of particular projects. We publish the "modified" coordinates, and provide all of the infomration to convert the date to a state-plane grid. If someone wanted to take a project and convert it, they could; and match it up to an adjoining project using the same procedure with that project's metadata.

Good idea? I'm not sure. Our engineers want to use cartesian coordinates to design from. Colorado's lowest elevation in the State is higher than the lowest elevation in any other state.....so, I am saying the whole state is above 3,300' (or so). There is a difference seen in the inverse of distances of any significant length anywhere in this state.

I definitely don't like the idea of holding one coordinate and making all of the others "radiate" from it (what I think you called "local" modified grid). It can look too much like state-plane values. It is our policy to use a multiplier and modify from 0,0 then truncate off the millions so as not to appear similar to State Plane values.

We have control points all over, and they are published in State Plane. A person can come off of one of our 'control points', use state plane, and convert a whole project including right-of-way to the state plane coordinates s/he is working in. Adversely, the person can tie to a right-of-way and use the metadata to convert his/her state plane coordinates on his/her control to the "modified" grid that the right-of-way plans is based on.

Just some thoughts.

Tom

Totally agree, Loyal (but then again, it seems you are I almost always do agree, at least on technical issues). There were very large projects done around here by a certain large engineering firm in the 80's that had that done, and it seemed like no one understood what they did, and the "metadata" mentioned by some here got lost in the sauce. Surveyors would try to bring in outside control as a check, and would go nuts trying to figure out why it didn't work. I had a couple of consulting gigs trying to figure out what was wrong. Easy Money!

I have been forced to supply a few jobs with scaled SPC (DOT), but I convinced them to whack a couple of digits off each coordinate so they didn't appear to be SPC.