A spin on the LDP and question

is gamma the mapping angle?

Gamma = Nad83
Theata = Nad27

That is twice. Did you misspell Theta on purpose or is it a new variable?

I don't show the angle except for a few plats where the state requires it, those drawings are one mile across and I show the angle to the nearest minute.

Is that a real scale factor?

I will round off the scale factor to 6 places. You are only talking about .005' in 10,000' so it really doen't mean much beyond the 6 place for distances.

No, I'm an idiot. 🙂

Mighty

Yes it's the CSF for the tract I'm working on. I remember Dave Doyle saying that 6 places was enough also, but then enter Loyal and he makes the argument that there are places where 6 ain't enough and he doesn't truncate. I suppose that while I could, especially in this instance, I put the data out there that I generate. That's what meta data is supposed to be, so I do it. 🙂

I usually define the convergence angle at a given point on the boundary, usually the point of beginning.

As for theata, that's one of those fancy European picture houses, right?

You guys suck. 🙂

Mighty

Well, it depends on what you are reporting.

If only distances are being considered then the 6 places are enough. And since you are not "fixing" the scale factor for the project but stating an average, going past the 6th or maybe the 7th place becomes a matter beyond any ability to measure.

Now, if we are talking about reducing coordinates then it matters. When you get into the millions, more places are important.

If it's a Tranverse Mercator LDP you would need to apply a scale factor. For those I get an average of the area I want to work in and then apply a 6 (or maybe 7) figure one. At that point I'm in control of the scale factor so I get to chose.

"It is a damn poor mind indeed which can't think of at least two ways to spell any word."
A. Jackson

Kris, you are working way to hard.

I believe you have Trimble gear.

So try this on one of your projects. Take one you already have in Trimble Business Center. Pick a point where you would like to see a LDP centered (a hard point you have a shot on at least you need a lat and long). Pick an elevation for your LDP Grid (ave project elevation nearest 20 feet or so). At your point enter a new point with the same horizontal coordinates but with your average elevation, call it ORIGIN. Under Report Options, Point List, set it to show all the factors (combined, height, projection). Pick your ORIGIN point and run the Points List Report. Take the Height scale factor and inverse it to get your LDP Scale factor (some modify this a bit more but for projects not wider than about 15 miles it doesn't make much difference). So now for your LDP you need the ORIGIN Lat, Long and Scale Factor which you now have (you only needed to get the scale factor). You also need to decide your false northing and easting. Now go to Project, Change Project Coordinate System. Select New System and click Next. Select Default Projection (Transverse Mercator) and click Next. Fill in your Central Lat & Long, false northing/easting and the scale factor (your ORIGIN point). Click next. Select your Geoid model and click finish. Recalculate your project (usually automatic). You can do a check by running the Points List Report to see if your factors are correct on point ORIGIN. The combined scale factor should be 1.00000000 for your Origin point, if not recheck your data.

So now check some of your LDP grid distances against your SPC x CSF distances. You could also inverse some hard ground distances and compare to the LDP grid distances. Check it out to your own satisfaction that it works or not. If your central meridian goes through the point you used to get your mapping angle you should be able to compare bearings, the difference should be at or very near your mapping angle.

So once you are done playing around and want your Texas SPC back just go to the Change Project Coordinate System and select the Texas SPC you had and everything will be back as before. You could even see what your coordinates are where I'm at (Utah Central Zone).

Should you wish to send me a check for all the time you are now going to save, email and I'll give you the address.

You can also set the Points List Report to give you the convergence angle for ever point (in any standard or LDP projection).

"So, the big question is, how do you do it for these types of projects."

Well - to start with, I would never even dream of showing grid data on a cadastral survey plat. NEVER. I know so few people that even have the faintest clue regarding grid data that I would almost sooner hand them a ticking time bomb...

Sad, but true.

Well, are some typical examples of the notes I use. They appear just after the preamble to the written description and before the coordinate list (which is also given before the metes and bounds are recited.

> 1. Bearings of Lines refer to Grid North of the Texas Coordinate System of 1983 (South Central Zone) as computed from GPS vectors,

While the reference to GPS vectors may seem unnecessary, Grid North can be computed from astronomical observations which is what I did in the past.

> 2. Distances are Horizontal Surface Distances in units of US Survey Feet, computed using a project average Combined Scale Factor of 0.999893
>Surface Distance = Grid Distance / 0.999893.

The Project Average is simply gotten by examining the Combined Scale Factors across the survey and looking in particular at the extremes and the average of all stations.

> 3. "Standard Rod and Cap" denotes a Punchmark on a 2 in. Aluminum Cap stamped "KENT MCMILLAN, SURVEYOR, RPLS, 4341" and numbered as noted, affixed to a 5/8 in. Iron Rod. “Standard Spike and Washer” denotes a Punchmark on a 3/8 in. Steel Spike with a 2 in. Aluminum Washer stamped "KENT MCMILLAN, SURVEYOR, RPLS, 4341" and numbered as noted.

If I tied a mess of someone else's rod and cap monuments, I usually give a shorthand phrase and full description for them here as well such as:

> "Morgan Rod and Cap" denotes a Red Plastic Cap imprinted "CONTROL POINT, NOT A BOUNDARY MARKER" affixed to a 3/4 in. Iron Rod.

and then,

> 4. Underlined Numbers appearing throughout this description are the Point Nos. appearing in the following list of coordinates. Coordinates are in units of US Survey Feet and refer to the Texas Coordinate System of 1983 (South Central Zone); NAD83 (CORS96) Epoch 2002.0 as derived from connections to the National CORS network via more than 5 hrs. of L1/L2 GPS observations on each of more than four days, processed using the National Geodetic Survey’s OPUS utility with rapid orbits. The coordinates in the following list were obtained by a combination of GPS and conventional methods and are estimated from analysis of variance to have standard errors in N and E components of less than +/-0.02 ft.

The things I care about are what the accuracy claimed for the results was and that the methods used weren't obviously inadequate to obtain that accuracy. So, I don't particularly care what day or days a connection to NAD83 was made via OPUS, but I do care how long the session was and if the connection was repeated on different days.

Mighty

> If only distances are being considered then the 6 places are enough. And since you are not "fixing" the scale factor for the project but stating an average, going past the 6th or maybe the 7th place becomes a matter beyond any ability to measure.

Right. I roll my eyes when I see an average Combined Scale Factor quoted beyond 0.1 ppm when surface distances were computed. 1 ppm is best practice as far as I'm concerned.

> To get geodetic bearings, rotate the bearings shown/recited, clockwise, by 02°38'22", which is an average for this project.

I'd never use that note since it's wrong unless the whole project lies on the same meridian of longitude or is a postage-stamp-sized parcel less than about 44 ft. to either side of the meridian where the equation hold true. Better practice would be something along the lines of :

> At the POINT OF BEGINNING, True Azimuth (Geodetic Azimuth) = Grid Azimuth + 2°38'22"

Map projections are a 2D model. We work with 3D data.

All of the advantages of a map projection and more are available when using the global spatial data model (GSDM). See http://www.globalcogo.com/LDPvsGSDM.pdf

Well said, LR. Some of my projections center on fixed points (particularly those we started some years ago, assigning 10,000/10,000 to an initial point), however most of my newer ones are on some even minute of lat and long near the project. Other than that, I think you've very simply stated the procedure. And, like you said, with most GPS software and even some of the CAD software available now, you can easily switch to whatever projection you want.

As for the bearing relation statement...
On small projects, I simply state, 'reported bearings are related to Geodetic North. To convert reported bearings (geodetic) to bearings related to Grid North for the Texas Coordinate System of 1983, North Central Zone, rotate reported bearings counter-clockwise by the mapping angle of 02°00'10"'. If the project is a bit larger, and as I have begun to use LDP's in-house, I've modified the statement to something like, ' reported bearings are related to a local grid having a central meridian of 94°50'00" W. To convert reported bearings (local grid) to bearings related to Grid North for the Texas Coordinate System of 1983, North Central Zone, rotate reported bearings counter-clockwise by the mapping angle of 02°00'10"'.

If someone chooses to publish their work in SPC, grid north, grid distances, that's fine. I can certainly follow well documented accurate SPCs.

> If the project is a bit larger, and as I have begun to use LDP's in-house, I've modified the statement to something like, ' reported bearings are related to a local grid having a central meridian of 94°50'00" W. To convert reported bearings (local grid) to bearings related to Grid North for the Texas Coordinate System of 1983, North Central Zone, rotate reported bearings counter-clockwise by the mapping angle of 02°00'10"'.

Actually, if the project is larger that the size of a postage stamp, meaning it extends more than about 88 ft. East and West of the Central Meridian of your Custom Lambert Projection, that statement would of course not be true. Need to reword it along the lines of "At the XYZ corner, my Custom Grid Bearings = True North (Geodetic North) + Mystery Angle."

Unless the standard parallels of your projection are the same as those of the SPCS North Central Zone, the odds are excellent that your Custom Grid North will not have an exactly constant relationship with Grid North of the SPCS North Central Zone everywhere.

Earl, I knew it would just be a matter of time before you posted on this thread 🙂

Your system solves all of the arguments going on in this and a couple of other threads this week!

SHG