Survey's plotted to Grid vs. Ground

1. No, no way to know without ground check or checking with who ever prepared survey.

2. Depends on methods used or preferences of surveyor preparing surveys.

3. The only way know is to determine the combined factor (scale factor X elevation factor) to convert the grid survey to ground, assuming site is fairly flat,

1. No. Sometimes survey performed on the grid are "scaled to ground" for mapping. Nobody likes that and some here really hate it. But some agencies mandate it. So it happens. IMO, there should be a statement on the survey explaining the control basis. Even if its local 5000,5000 so state.

2. Lots of work still being done on the 5000,5000 local system here in the PNW. IN OK lots of RTK work was done with autonomous base positions so the coordinates were grid +/- 20 feet or so.

3. Unless you intend to compare apples to bananas something is going to have to be converted. As far as which one, that depends on the available data and the needs of the project.

rfc, post: 336093, member: 8882 wrote: It's Friday, so I allow myself the freedom to ask the dumb questions the answers to which I haven't found anywhere else yet....
The answers are probably easy for one who's looked at a gazillion surveys (that's not me), so here goes:

1. If a survey is plotted with bearings referencing Grid North, can it be assumed that the distances shown on such a survey would be grid distances?

2. Are modern surveys more typically plotted using Grid coordinates, vs. older (or ancient) surveys, that were all on the ground?

3. If one wants to work completely on ground, and wishes to use portions of another survey (for reference), that's been done on Grid, is it appropriate to convert the survey to ground first for integration?

What type of surveys are you asking about? Boundary, topographical, engineering design, etc. ... I personally would never prepare a boundary or a topographical survey on anything but ground...

rfc, post: 336093, member: 8882 wrote: 1. If a survey is plotted with bearings referencing Grid North, can it be assumed that the distances shown on such a survey would be grid distances?

No, absolutely not. Hopefully they have an explanation on the plat what is what. Most people who convert to a project database, use scale factors but don't rotate to a different north basis.

2. Are modern surveys more typically plotted using Grid coordinates, vs. older (or ancient) surveys, that were all on the ground?

No. Again they should be telling you what they are doing on the plat.

3. If one wants to work completely on ground, and wishes to use portions of another survey (for reference), that's been done on Grid, is it appropriate to convert the survey to ground first for integration?

Do what you need to do in order to work in your bearing/coordinate base. You need to have the "metadata" to know how to utilize coordinates in another coordinate base. Rotating and translating might be the way to go, but be very careful to make sure you do it right. Do some inversing before and after for checks. You don't want to rotate or translate to two totally different points. Measure in some common points in the other database, so that you can do your conversion and check between common points to make sure you are in a common database.

Tom Adams, post: 336098, member: 7285 wrote: No, absolutely not. Hopefully they have an explanation on the plat what is what. Most people who convert to a project database, use scale factors but don't rotate to a different north basis.
No. Again they should be telling you what they are doing on the plat.
Do what you need to do in order to work in your bearing/coordinate base. You need to have the "metadata" to know how to utilize coordinates in another coordinate base. Rotating and translating might be the way to go, but be very careful to make sure you do it right. Do some inversing before and after for checks. You don't want to rotate or translate to two totally different points. Measure in some common points in the other database, so that you can do your conversion and check between common points to make sure you are in a common database.

Good info, all. This is a boundary survey. In the "Notes" section it says "Bearings shown on this plat refer to Vt Grid North, as established by OPUS observations...Boundaries are computed from a Topcon yada yada Total Station field survey"

That would imply Grid bearings, ground distances, no?

And to your advisory on incorrectly using two different (but apparently in the same place) points: Yup. Been there, done that already. :pissed:

If I saw distances, I would assume they are the horizontal distances as measured @ ground level unless they tell me otherwise; especially on a boundary survey, and If it says "grid north" I would assume they mean "State Plane Grid" unless they specifically state some other "grid"

If the map has no metadata, check the title block and get on the phone. It never hurts to ask...

thebionicman, post: 336116, member: 8136 wrote: If the map has no metadata, check the title block and get on the phone. It never hurts to ask...

And that is when you hope they actually know what they did, you would be surprised at how often they don't know...

If it's on the grid, keep it on the grid. This is my standard meta-data statement about coordinates, bearings, and distances. I've gone both ways and finally figured out just keep it on the grid.

Bearings are based on the Texas Coordinate System of 1983, Texas Central Zone per GPS observations. All coordinates are U.S. Survey Feet, NAD83(2011) Epoch 2010.0000 per static GPS data and an OPUS solution through the NGS website. To get geodetic bearings, rotate the bearings shown hereon by the average angle of convergence, clockwise, 02å¡30'22". All distances are grid and to get surface distances, divide the distances shown hereon by the average combined scale factor of 0.99997898.

"BEARINGS REFER TO GRID NORTH" is dangerous. Possibly only one bearing is exactly on grid north and the survey map preparer may think all are but if he/she failed to account for convergence the rest may be wrong.

Given a map referenced to grid with at least 3 points with grid coordinates you may be able to figure it out in the office.

OR

It my take you the time to confirm both angle and distances as well as coordinates in the field.

The more thorough the map coordinate explanation the greater your chances at success. But there can be no shortcut to your thorough knowledge of surveying systems.

Paul in PA

Shelby H. Griggs PLS, post: 336123, member: 335 wrote: And that is when you hope they actually know what they did, you would be surprised at how often they don't know...

Completely agreed. The call may not get the Metadata but it will get you started down the right path. That path may just be completely around the map in question...

Great topic, thanks for getting it started rfc.

kjypls, post: 336166, member: 9749 wrote: Great topic, thanks for getting it started rfc.

No problem! As you may know, I'm attempting to learn surveying, without the niceties of school or working for an LPS.

I feel it's been like walking into a labyrinth of knowledge...thousands of passage ways, each holding a nugget of knowledge. Most are of casual importance, each by itself not critical, but taken together build a broad base of understanding.

But others hold the jewels to making it all work; without which you'd be spinning your wheels for a long time.

The problem, though, is that each passage way looks identical to all the others, and the further in you get, the more they look the same. Many just dead end, and you have to retrace, backtrack, and try again. Asking questions here is like poking into what looks like a shallow alcove, only to find endless additional offshoots. The perspectives from the folks here really expand the areas I'd like to investigate further. Sometimes it really is a challenge to know on which one should focus ones energies.

I'm enjoying that challenge though.:-)

rfc, post: 336093, member: 8882 wrote: It's Friday, so I allow myself the freedom to ask the dumb questions the answers to which I haven't found anywhere else yet....
The answers are probably easy for one who's looked at a gazillion surveys (that's not me), so here goes:

1. If a survey is plotted with bearings referencing Grid North, can it be assumed that the distances shown on such a survey would be grid distances?

2. Are modern surveys more typically plotted using Grid coordinates, vs. older (or ancient) surveys, that were all on the ground?

3. If one wants to work completely on ground, and wishes to use portions of another survey (for reference), that's been done on Grid, is it appropriate to convert the survey to ground first for integration?

I've never heard or seen State Plane Coordinates or State Coordinate System Coordinates refered to as "grid" on a plat, a description, or even by the surveyors I've worked with, it seems to be a new thing, slang and lazy speech. Using it on a plat would be a meaningless statement and frankly unprofessional.

I've seen surveys that stated that the bearings were state plane bearings or State Coordinate System bearings.

But, there are an infinite number of grids available, State Coordinate Systems aren't the only ones.

I have seen surveys that stated that the bearings were grid and that grid bearings=geodetic bearings at a longitude.

Almost every survey will be grid of some kind, it's unusual to find a real True North survey outside of the BLM.

And I always assume that any survey will have ground distances, unless otherwise stated.

But then I work at elevation.

MightyMoe, post: 336204, member: 700 wrote:
But then I work at elevation.

Does that mean that you work "on ground"...All elevations are ground elevations? Or that you work at some defined "mean project elevation"?

1. Not safe to make such an assumption = don't make it.

2. All of our work is provided in Ground, unless specified otherwise by the client/project needs.

3. As long as sufficient control is connected so that the calculations can be made; Sure, convert away. If there isn't enough control you can still work with it, but you have to know the limitations of that data.

rfc, post: 336211, member: 8882 wrote: Does that mean that you work "on ground"...All elevations are ground elevations? Or that you work at some defined "mean project elevation"?

No I don't work at a defined elevation, I will work using a DAF, datum adjustment factor, it's expressed as a number larger than one, and it's figured out by dividing ground distance across a site by the grid distance, I completely disregard any elevations or scale factors. What you want is a distance as close to ground as is possible anywhere in the area you are surveying.

This is one I'm working on today, from the NW point of the survey to the SE point the working distance is 17,016.78, the actual ground distance between those two points is 17,016.81, the DAF is 1.000205. The idea is to simulate the ground distance as well as you can, differences in elevation will make it impossible to be perfect with one DAF. In other areas my working ground distance will be slightly larger than the actual ground distance. It will probably never calculate exactly between two points.

You may think of it as raising the ellipsoid up to a new elevation, but that really isn't what you are doing, because state coordinates also have scale factors applied.

However most surveys that I retrace were done before any GPS and they are usually measured ground distances between points, no elevation considered, no scale factors even if they were on a state plane system. If they are state plane bearings, usually they occupied a state plane monument and surveyed from there using what they measured and adjusted into another monument. Very few of these would have been reduced to state plane, that was normally only done for large mapping projects, almost never for boundaries. There just wasn't any good reason to use state plane distances for these surveys, but state plane bearings are very useful.

Using a DAF is simply applying the CAF to an existing projection. You are still using the scale and Elevation factor. They are incorporated into the parameters rather that being applied after. I've been using that method since the 90s..

rfc, post: 336168, member: 8882 wrote: No problem! As you may know, I'm attempting to learn surveying, without the niceties of school or working for an LPS.

I feel it's been like walking into a labyrinth of knowledge...thousands of passage ways, each holding a nugget of knowledge. Most are of casual importance, each by itself not critical, but taken together build a broad base of understanding.

But others hold the jewels to making it all work; without which you'd be spinning your wheels for a long time.

The problem, though, is that each passage way looks identical to all the others, and the further in you get, the more they look the same. Many just dead end, and you have to retrace, backtrack, and try again. Asking questions here is like poking into what looks like a shallow alcove, only to find endless additional offshoots. The perspectives from the folks here really expand the areas I'd like to investigate further. Sometimes it really is a challenge to know on which one should focus ones energies.

I'm enjoying that challenge though.:-)

"...without the niceties of school or working for an LPS."

One or the other of those is a necessity, not a nicety.