Activity Feed › Discussion Forums › GNSS & Geodesy › LDP (projections)

LDP (projections)
Posted by Tom Adams on April 8, 2016 at 4:29 pmHey Loyal, or anyone, can you give a brief explanation of what it is or how to generate a Low Distortion Projection (Is that the right terminology) Are there simple softwares to use? Can you explain to the nonfamiliar surveyor in layterms what it is? Reference a book or reading material that talks about it and/or how to generate it?
Thanks,
Tom (an LDPChallenged surveyor)Tom Adams replied 8 years, 3 months ago 9 Members · 34 Replies 
34 Replies

try going to the Oregon LDP site for a wealth of info.
http://www.oregon.gov/ODOT/HWY/GEOMETRONICS/Pages/ocrs.aspx 
Tom,
You should check out Shawn Billing’s articles:
http://www.amerisurv.com/PDF/TheAmericanSurveyor_BillingsGroundVersusGridLDPpart1_Vol10No9.pdf
http://www.amerisurv.com/PDF/TheAmericanSurveyor_BillingsGroundVersusGridLDPpart2_Vol10No10.pdf
There is also a NGS Powerpoint that you can find by Google’n “Low Distortion Projection,” along with links to other articles and information on the subject.
The one linked by John is a good one also.
Loyal

Thanks John, I would like to know more about the mechanics of it, but that does have a brief explanation and a list of what its advantages are. (Maybe I’ll navigate around in there to see if there is more information on it)

Loyal, post: 366227, member: 228 wrote: Tom,
You should check out Shawn Billing’s articles:
http://www.amerisurv.com/PDF/TheAmericanSurveyor_BillingsGroundVersusGridLDPpart1_Vol10No9.pdf
http://www.amerisurv.com/PDF/TheAmericanSurveyor_BillingsGroundVersusGridLDPpart2_Vol10No10.pdf
There is also a NGS Powerpoint that you can find by Google’n “Low Distortion Projection,” along with links to other articles and information on the subject.
The one linked by John is a good one also.
Loyal
Awesome, thanks Loyal.

along the lines of what Loyal is saying, ngs has a couple good sources, some of which have mike dennis’ (sp) name attached. i will dig up some other resources, but they are at the hacienda, so check again this weekend.

So if somebody has designed an LDP and I have latlon points that I want to express in it, but don’t have a major software package, how do I convert?
I think I understand the general concepts, but haven’t researched the practical details. Are there tools I should be finding on line or formulas to put in a spreadsheet?
. 
Bill93, post: 366299, member: 87 wrote: So if somebody has designed an LDP and I have latlon points that I want to express in it, but don’t have a major software package, how do I convert?
I think I understand the general concepts, but haven’t researched the practical details. Are there tools I should be finding on line or formulas to put in a spreadsheet?
Bill,
the formulae, equations, computer code, etc. for converting Lat/Lon positions to “LDPs” is EXACTLY the same as used for State Plane & UTM conversions. The ONLY difference, is in the projection parameters (just as the parameters for UTM Zone 11, are different than Nevada SPC East (or West, or whatever).
USGS Professional Paper No. 1395 (which I linked a couple of days ago), has the requisite data, for all of the commonly used projections (Transverse Mercator, Lambert Conical, Oblique Mercator [Hotine]).
Loyal

Loyal, post: 366301, member: 228 wrote: USGS Professional Paper No. 1395 (which I linked a couple of days ago), has the requisite data
I got a spreadsheet working with those formulas to compute Iowa Regional Coordinate System northing and easting from latlon in the local zone (out of 14 zones in the state). It matches a few values from their handbook to 0.0001 ft.
I plan to convert my hobby measurement project (that uses towers and GPS points spread over several miles) from SPC to this system and see if I notice any difference in fit.
Stupid question #N+1 : I think a LDP has less arctochord correction by having the standard parallel and meridian close to your project. Is that true? I haven’t digested the formulas well enough for that yet.
Side comment: I’m old enough to have computed things with trig and log tables, slide rules, and mechanical calculators. I can’t imagine doing those coordinate conversions without a computer program.
. 
“Stupid question #N+1” – LOL
SQ N+2
So it sounds like you are coming up with a little more specific multipliers for your specific sight. Do you still keep you bearings the same as the SP Grid bearing base Parallel to the origin longitude? or do you rotate your bearings to appear closer to what they are @ your longitude? I see in Shawn’s article example that he was tying to a previous job that had used an astronomic observation.SQ N+3 (&3.5)
Would one of you guys who as worked in an LPD mind showing an example of a plat you have created and what kind of “metadata” (or what kind of information on your plat) you publish to show exactly how someone can get on the same database if they use gps? Do you publish the SPCoordinates for your various points?I haven’t read all of the links yet, so I’ll try to glean more information from there. I did read Shawn Billings’ articles and will reread those.
I searched SurveyorConnect for “USGS Professional Paper No. 1395” but didn’t find that link. But I did a googlesearch and found it here: http://pubs.usgs.gov/pp/1395/report.pdf if someone wants to use it.

Bill93, post: 366395, member: 87 wrote: I got a spreadsheet working with those formulas to compute Iowa Regional Coordinate System northing and easting from latlon in the local zone (out of 14 zones in the state). It matches a few values from their handbook to 0.0001 ft.
I plan to convert my hobby measurement project (that uses towers and GPS points spread over several miles) from SPC to this system and see if I notice any difference in fit.
Stupid question #N+1 : I think a LDP has less arctochord correction by having the standard parallel and meridian close to your project. Is that true? I haven’t digested the formulas well enough for that yet.
Side comment: I’m old enough to have computed things with trig and log tables, slide rules, and mechanical calculators. I can’t imagine doing those coordinate conversions without a computer program.
Calculating between lat, long and N, E wasnt’ too bad, there was a cheat sheet in the old booklets for NAD27, I did have a calculator, but not a program, I think you wouldn’t have much trouble, that being said I think last one I did longhand was 1979 or 80.:$

I still need to get the LDP convergence angle equattions added so I can keep the Polaris sights in the adjustment. There are few distance measurements and they are short so scale factor is not critical.
. 
Tom Adams, post: 366417, member: 7285 wrote: “Stupid question #N+1” – LOL
SQ N+2
So it sounds like you are coming up with a little more specific multipliers for your specific sight. Do you still keep you bearings the same as the SP Grid bearing base Parallel to the origin longitude? or do you rotate your bearings to appear closer to what they are @ your longitude? I see in Shawn’s article example that he was tying to a previous job that had used an astronomic observation.SQ N+3 (&3.5)
Would one of you guys who as worked in an LPD mind showing an example of a plat you have created and what kind of “metadata” (or what kind of information on your plat) you publish to show exactly how someone can get on the same database if they use gps? Do you publish the SPCoordinates for your various points?I haven’t read all of the links yet, so I’ll try to glean more information from there. I did read Shawn Billings’ articles and will reread those.
I searched SurveyorConnect for “USGS Professional Paper No. 1395” but didn’t find that link. But I did a googlesearch and found it here: http://pubs.usgs.gov/pp/1395/report.pdf if someone wants to use it.
This may not be what you are interested in, for this job, like almost every job where I used LDP’s there were no coordinates delivered or published. Only LAT, LONG, elevations, bearings and distances:

Thanks MightyMoe, Do you publish additional lats and longs, or SPCoordinates, and or, project specific coordinates? Or simply the bearings and distances. I trust it’s still on a State Plane Bearing base? You don’t adjust bearings of lines based on convergence angles I am guessing. What does you BOB statement look like?
Also, do I automatically know what your “combined scale factor” is by definition? A lot of guys around here use a factor that would convert a SP distance to a ground asmeasured distance which is always greater than 1 (since the lowest elevation in this State is above 3,300 feet. However, I see the definition of combined factor as being the factor that would take you from a ground distance to SP, which would typically be 0.999XXX.
I have seen a few articles or teaching programs, that use the term to go from SP to ground as being the “Ground Scale Factor” to differentiate it from “Combined scale factor”. I know this if offtopic of LDP a bit, but, being kind of picky, just wondering. I almost think it would be better to publish the “Grid Scale Factor” and the “Height Scale Factor”


Another question. Our crews use the Scale factor that the Trimble TBC software spits out. I can never reproduce the same values using the standard 20,906,000ft, or the 6,372,200M, of course this is insignificant for distances, but the “Combined Scale factor” is a lot different than if you used the “ellipsoid Radius” @ your particular Lat/Long. Not that we care that much for a LDP, but we need to either “force” the software to use the rounded Scale Factor, or we need to take a different Radius into account.
Of course we are using the typical DOT“Bastardized” coordinate system, so we are making faux “modified” State Plane Coordinates”. We need to show these values.
Just some thoughts on the subject. From my experience our “bastardized” coordinates seem to reflect ground distances and angles pretty good.
Sorry for all the questions, but I am certain that there are a lot of guys here that have some of my same questions. (And I’m sure a lot of these questions have been addressed before)

Loyal, post: 366457, member: 228 wrote: This is an interesting topic, and I wish that I had more time this weekend to get into the discussion (when is there ever enough time?).
Here’s a “cutnpaste” from a recent ROS Plat, that defines the LDP used.
Loyal
Awesome! Thanks Loyal. That helps a lot. I’m sure that I can’t get any DOT’s to change, but I certainly am going to play around with this some more. Maybe I’ll convert one of our existing jobs to an LPD and kind of see what kinds of differences I get. If I can understand it better, I can discuss it better (duhhhh….no sh** sherlock)

Tom Adams, post: 366459, member: 7285 wrote: Another question. Our crews use the Scale factor that the Trimble TBC software spits out. I can never reproduce the same values using the standard 20,906,000ft, or the 6,372,200M, of course this is insignificant for distances, but the “Combined Scale factor” is a lot different than if you used the “ellipsoid Radius” @ your particular Lat/Long. Not that we care that much for a LDP, but we need to either “force” the software to use the rounded Scale Factor, or we need to take a different Radius into account.
Of course we are using the typical DOT“Bastardized” coordinate system, so we are making faux “modified” State Plane Coordinates”. We need to show these values.
Just some thoughts on the subject. From my experience our “bastardized” coordinates seem to reflect ground distances and angles pretty good.
Sorry for all the questions, but I am certain that there are a lot of guys here that have some of my same questions. (And I’m sure a lot of these questions have been addressed before)
Tom,
Yeah, the various “radii” related to a specific Latitude does make a [small] difference. For example:
PID MR0820 EVANSTON
Latitude 41Ã¥Â¡15'17.06488" North NAD83 GRS80 Ellipsoid
Traditional Rc = 20,906,000.000 (Ft) 6,372,161.544 (M)
NORTHSOUTH Rc = 20,876,606.953 (Ft) 6,363,202.526 (M)
EASTWEST Rc = 20,956,126.701 (Ft) 6,387,440.193 (M)
MEAN Rc = 20,916,366.827 (Ft) 6,375,321.360 (M)
Geometric Rc = 20,916,329.037 (Ft) 6,375,309.841 (M)
Gaussan Sph.Rc = 20,916,404.617 (Ft) 6,375,332.878 (M)
Ellipsoid Height 2,126.66 meters
Vc = 0.999 666 369 1/Vc = 1.000 333 742 Traditional Rc
Vc = 0.999 665 899 1/Vc = 1.000 334 212 North/South Rc
Vc = 0.999 667 167 1/Vc = 1.000 332 944 East/West Rc
Vc = 0.999 666 534 1/Vc = 1.000 333 577 Mean Rc
Vc = 0.999 666 534 1/Vc = 1.000 333 578 Geometric Rc
Vc = 0.999 666 535 1/Vc = 1.000 333 576 Gaussen Rc
Vc = 0.999 666 53 1/Vc = 1.000 333 58 NGS Value
Rc = Radius of Curvature
Vc = Vertical Coefficent ("elevation factor")
Loyal

My bigger point is that using the conventional radius vs. the ellipsoid radius, makes a bigger difference in the Combined Scale Factor than whether or not you use the “Grid Scale Factor” @ all. ie: the GSF is less significant than the approx. vs. calculated HeightScale Factor.
(or maybe I’m wrong. I’m just stating from memory)
The secondary point is that you should know what your software is using and how to extract it, if you are letting the software generate the csf.

Bill93, post: 366395, member: 87 wrote:
Stupid question #N+1 : I think a LDP has less arctochord correction by having the standard parallel and meridian close to your project. Is that true?
Yeah…I guess…I’ve never really thought about it!
Considering the “postage stamp” nature of most LDPs relative to the “Beach Blanket” nature of SPC Zones (not to mention UTMs or Montana), the difference between the “arc” and the “chord” doesn’t amount to much.
First, one has to get the “PLANE” in State Plane out of their head to start with.
The State “Plane” Coordinate System(s) are NOT “PLANES” in the geometric sense, but 3d developed surfaces that “can be laid out” FLAT (as on a PLANE surface).
In the case of a Transverse Mercator, the [NorthSouth] “arcchord” issue is essentially moot, in that the developed surface is [in effect] a cylinder. The Lambert developed surface is a cone. The EastWest (Transverse Mercator) or NorthSouth (Lambert) arcchord variances are of course dealt with via the [grid] Scale Factor (k) progression. This gets a little weirder when dealing with Oblique Mercator (Hotine) projections.
Of course “tangent planes” are another issue.
I hope that made sense…
Loyal
Log in to reply.