MKennedy:
Specifically, what it meant by "may not correspond well with the terrain surface in a location (AKA grid versus ground)"? Please provide a definition or example of "correspond well".
Isn't the difference between grid and ground simply the difference?
DWoolley
DWoolley, post: 374818, member: 6749 wrote: MKennedy:
Specifically, what it meant by "may not correspond well with the terrain surface in a location (AKA grid versus ground)"? Please provide a definition or example of "correspond well".
Isn't the difference between grid and ground simply the difference?
DWoolley
Well what we are really talking about, is the "developed surface" (grid surface) of one projection (say State Plane or UTM) verses the "average" topographic surface of a given project.
For example: (readers digest version)
The Utah State Plane Coordinates have a "developed surface" (grid surface) ranges from about 2000 feet BELOW the Ellipsoidal surface, to about 2000 feet ABOVE the Ellipsoidal surface. There is no where in Utah that the topographic surface intersects the Ellipsoidal surface (or even 2000 feet above it). (well maybe the Virgin River Canyon in extreme SW Utah)
So, when you are working at [say] 5 or 6 thousand feet above the Ellipsoid (pretty common), the "surface" of the SPC grid is ABOUT a mile BELOW you (it of course varies depending on where you are located relative to the central parallel of the SPC Zone you are in).
An LDP creates a "developed surface" (GRID surface), that matches the average ellipsoid height of your project.
Loyal
I assume that smaller mapping angles are another benefit of LDP's?
DWoolley, post: 374785, member: 6749 wrote: It is my understanding there is an LDP in San Francisco City/County.
After giving Low Distortion Projections some thought, reading Shawn Billings' two part article a while back, I do not understand the purpose or gain realized by creating a LDP. Grid is always grid.
What am I missing?
DWoolley
And ground is always ground - and that is where I find myself measuring... every time. Can't get to the "grid" from here, it's 7,000 feet below me, and I've really never found much use for it.
What am I missing?
I think many LDP's are strictly scaled spc, to make a local grid, that are very close to ground.
The vexing issues using state plane have always been bearings being so far from "true" bearings and distances usually being shorter than measured distances.
To "soften" the impact of state plane, or utm, or whatever canned system; LDP's have become more popular. I'm more interested in the bearing issue, so much of my work is dependent on original "true" bearings and I can simulate those in small east-west areas (township) using my own LDP.
The distance issue is a trivial one in State Plane, that's simple enough to deal with, and using a LDP doesn't really mean much because you need to develop a scale factor anyway so you are basically handling the distances in a LDP just like you do in State Plane.
Nate The Surveyor, post: 374868, member: 291 wrote: I think many LDP's are strictly scaled spc, to make a local grid, that are very close to ground.
Well Natster, I would say that would be "modified" State Plane Coordinates, not an LDP (by definition).
As Baja pointed out above, by using a formal LDP, one also aligns the GRID to be more consistent with the Cadastral Fabric (at least in the PLSS states).
There really isn't a silver bullet that slays all dragons (except maybe the PLSS Datum as described in the BLM Manual). Every solution has it's plus's and minus's, unfortunately, far too many surveyors fail to grasp the underlying geometry in play.
The "State Plane" isn't really a "PLANE," but it can be treated as such for most practical applications. LDP "grid" isn't either.
Loyal
The basis of a state plane coordinate system is to relate a precise model (ellipsoid/spheroid), expressed in terms of latitude and longitude, to a corresponding rectangular plane coordinate system. The relationship between the two surfaces is the "projection". There is a defined and published mathematical pattern i.e. zone constants which accounts for gravity, latitudinal distortion, elevation etc. The zones, when accounting for the various factors, are designed to create a unity between measured values on the ellipsoid and the earth's surface - in NAD27 this was expressed as a ratio of 1:10k. The zone constants allowed surveys on the SPC system to be accurately interrelated over large areas and maintain a precise relationship between the latitude and longitude and the rectangular system - for the purpose of this example we'll call it "scale". Given true SPC based on the published zone constants, regardless of the amount of area or position within the zone, allowed the surveyors to precisely and uniformly work within the system.
Of course, the readers know this information.
The LDP, as I understand it, is a hybrid system - not SPC or an assumed ground system- created to reduce the scale on a local project. Again, why? If the different between grid and ground is 0.1' per 1000' or 0.3' per 1000'- why is one more preferential than the other? Wouldn't an LDP create a third untraceable system with 0.2' per 1000' - disguised as SPC? The same can be said for the mapping angle - if it is "constant" within a local area what difference is it to a surveyor whether it is a few minutes either way? It is still a matter of addition or subtraction.
The SPC and the related zones are understood by land surveyors and published - oftentimes legislated. Any surveyor can move in and out of the system accurately without any unpublished distortion. The surveyor is either in grid or a calculated ground system.
I would appreciate any responses to my questions.
Baffled,
DWoolley
PS Last question, if working in a large area why would a surveyor move the move off SPC? Conversely, if working in a small area, why would a surveyor move off SPC?
In Wisconsin and I would think this is the rule rather than the exception that a lot of the LDP's are developed by DOT. I think the reason for this is to make grid to ground negligable enough that you can use grid as ground to simplify the process of survey to design to construction.
Loyal, I humbly acknowledge.
Ya know, surveying ain't simple no more.
N
DWoolley, post: 375039, member: 6749 wrote:
Wouldn't an LDP create a third untraceable system with 0.2' per 1000' - disguised as SPC?
It's not untraceable if it's documented, which is exactly what you're supposed to do. In an ideal world, a LDP will also have coordinates that are unlikely to be confused with SPC or UTM through careful choice of the projection parameters.
Do people apply a grid-to-ground adjustment to SPC coordinates and still call it SPC? Yep.
Melita
There seems to be some angst in the minds of some about LDP's. There's really no need for it. When state plane systems were developed it was not anticipated that coordinates would ever be modified to ground. The systems were developed to work with the measurement capabilities used for most applications making the assumption that grid distances were ground distance for all intents and purposes. Measurements have improved and LDP's are designed for the very same purpose that state plane systems were. That is that grid and ground are the same for most applications. I can assure you taking away the need to work with scale factors simplifies many processes. You call up the system you are in programmed into your field or office software of choice and that's it. No need to have separate coordinate files, GPS and total station observations work well together. If statewide LDP's are designed appropriately the user can tell what the system is by simply noting the coordinate. It can't be duplicated in any other state system. In our system if the easting is 14 million + the user knows it is zone #4. 20 million + = zone 10 etc.
As surveyors we are bothered because we know grid is never ground. LDP design involves getting stakeholders together and agreeing on the maximum ppm in linear error that can be lived with and design the LDP based on that, political boundaries, high population areas, etc.
DWoolley, post: 375039, member: 6749 wrote: The basis of a state plane coordinate system is to relate a precise model (ellipsoid/spheroid), expressed in terms of latitude and longitude, to a corresponding rectangular plane coordinate system. The relationship between the two surfaces is the "projection". There is a defined and published mathematical pattern i.e. zone constants which accounts for gravity, latitudinal distortion, elevation etc. The zones, when accounting for the various factors, are designed to create a unity between measured values on the ellipsoid and the earth's surface - in NAD27 this was expressed as a ratio of 1:10k. The zone constants allowed surveys on the SPC system to be accurately interrelated over large areas and maintain a precise relationship between the latitude and longitude and the rectangular system - for the purpose of this example we'll call it "scale". Given true SPC based on the published zone constants, regardless of the amount of area or position within the zone, allowed the surveyors to precisely and uniformly work within the system.
Of course, the readers know this information.
The LDP, as I understand it, is a hybrid system - not SPC or an assumed ground system- created to reduce the scale on a local project. Again, why? If the different between grid and ground is 0.1' per 1000' or 0.3' per 1000'- why is one more preferential than the other? Wouldn't an LDP create a third untraceable system with 0.2' per 1000' - disguised as SPC? The same can be said for the mapping angle - if it is "constant" within a local area what difference is it to a surveyor whether it is a few minutes either way? It is still a matter of addition or subtraction.
The SPC and the related zones are understood by land surveyors and published - oftentimes legislated. Any surveyor can move in and out of the system accurately without any unpublished distortion. The surveyor is either in grid or a calculated ground system.
I would appreciate any responses to my questions.
Baffled,
DWoolley
PS Last question, if working in a large area why would a surveyor move the move off SPC? Conversely, if working in a small area, why would a surveyor move off SPC?
Not to be rude, but you are both baffled and ignorant.
I'm typing this from the Portland International Airport in Portland, Oregon. The entire State of Oregon is now covered with LDP's. With these projections, the vast majority of the State has <20ppm scale distortion between the surface of the Earth and the projection surface. It's important to reiterate (since it's already been stated) the 1:10,000 (or 100ppm) that was a design criterion for the original SPC system was for the difference between the ellipsoid and the projection surface. The Earth surface (topography) never entered into the design. Gravity never has either (as your post suggests).
From the NGS advisor's presentation yesterday, the Oregon DOT was using modified SPC for highway projects for the past 70 years (prior to the ORCS (Oregon Regional Coordinate System) being developed. If that's true (and I'm sure it is) then the DOT was already trying to resolve the scale difference between surface and Grid almost as soon as the SPCS was developed! An LDP is EXACTLY like a SPCS in construction. Same projections, same constants, except that the distortion is minimized in the design such that an inverse distance on the projection surface is roughly equivalent to the measured horizontal distance at the Earth surface. There is still distortion in an LDP, hence the Low instead of No. In Oregon, a great deal of the State has combined factors that are nearly 300 ppm. This simply can't be ignored. And while there are solutions to this, each of them has its issue. The only drawback to LDP that I'm aware of is that they are often not readily published. This can be resolved though.
With the 2022 reference frame coming, I hope that more States (including Texas) will develop LDP's for the entire State in anticipation. This is a tedious issue that is not only technical but also legislative. With the new reference frame, the State Coordinate Systems (State Plane) will need to be revised so that 2022 coordinates look different from the 1983 coordinates. Since the legislative effort will be the same, whether changing the SPCS or starting with a new LDP system, we may as well start moving forward.
