Low Distortion Projections

 

Which would you rather see?

 

“all these here big numbers are _____ State Plain(sic) Coordinates.”

 

No Datum, Epoch, Realization, or even Units... nadda! No indication of HOW, From WHERE, under what circumstances, or what techniques were used to generate these “big numbers.” WORTHLESS...plain and simple!

 

Or a plat, ROS, Report, or description that actually tells you something about the “coordinates,” and how they were determined?

 

Something like:

 

Datum NAD83(Cors96)

Epoch 2002.0000

Linear Units U.S. Survey Feet

Projection Transverse Mercator

Central Meridian xxxºxx' xx.00000” West

Origin Latitude yyºyy' yy.00000” North

Scale Reduction 1.000 xxx xxx Unitless

False Northing nnn,nnn.nnn US Survey Feet

False Easting ee,eee.eee US Survey Feet

Design Elevation u,uuu.uu NAVD88 (feet)

Average Geoid (N) -NN.N meters

 

Primary Control Stations (Point Numbers 1 & 2) where simultaneously observed on July 5^th and again on July 7^th for a minimum of four hours on each day. These Static Observations were processed to Plate Boundary Observatory (PBO) Stations P007, p086, & P105 using the NGS OPUS_S program, and refined based on the NGS 60 Day Times data prior to adjustment in a Least Squares Network Adjustment, holding the the following ITRF2000 Epoch 2010.xxxx Coordinate estimates as partial fixities:

 

P007 X... Y... Z... rmsX x.xxxx rmsY y.yyyy rmsZ z.zzzz

P086 X... Y... Z... rmsX x.xxxx rmsY y.yyyy rmsZ z.zzzz

P105 X... Y... Z... rmsX x.xxxx rmsY y.yyyy rmsZ z.zzzz

 

HTDP 3.0 was utilized to transform the adjusted ITRF2000 Epoch 2010.xxxx Coordinate estimates to NAD83(CORS96) Epoch 2002.0000.

 

NAD83(CORS96) Epoch 2002.0000 Coordinates estimates are as follows:

 

Pt.No.1 SE07-GLO X... Y... Z... rmsX x.xxxx rmsY y.yyyy rmsZ z.zzzz

Pt.No.2 E170-NGS X... Y... Z... rmsX x.xxxx rmsY y.yyyy rmsZ z.zzzz

 

All Stations, Corners, and Monuments shown hereon where observed by Static, Real-Time-Kinematic (RTK), or Conventional Total Station techniques, and constrained to the Primary Control Stations above.

 

Now that's just a quick-n-dirty example, but something along those lines would really tell you something (and take only a few extra minutes to generate).

 

The ONLY part of the above blurb that would be UNIQUE to an LDP (or other “user defined coordinate system”), or ADDITIONAL compared to using SPC or UTM, would be the portion in red. In fact, only the first six lines of that are REALLY important.

 

I would submit that the blue portion SHOULD be a bare minimum for SPC or UTM (when using OPUS). A simple re-write of the first paragraph (in blue) would cover usage of the NGS Passive Network.

 

Dats my 2 bits,

Loyal

I doubt we are as far apart as the last posts imply. If a surveyor (or other geo-magician) cannot publish metadata they should not publish coordinates. It's not a matter of what I prefer to get, it's a matter of what we often see. The 'disagreement' (if you can even call it that) is the value of coordinates given sub-standard metadata.

I never use coordinate data without verifying it first. Time has taught me to be just as suspicious of pretty statements as I am of lists with no explaination. I can't tell you how many times I've seen certain companies slap the same Basis of Survey statement on every survey they publish. It is obvious that the only person who knows what it says was the tech who wrote it for one specific survey. It sounds cool so it gets plastered on everything. The work to evaluate mystery coordinates isn't that much different than validating those with a complete Basis of Survey.

I am certain that we will improve in this regard over time. It will take time for the standards to develop, and even more time for Legislation and regulation to catch up. In the meantime we have to work with what we get, and assign it a proper value as evidence. Hopefully I will retire before absolute coordinates reach the upper end of the order of calls.

Ramble over, Tom

nmillerpls:
Argument 1: Go to LDP's similar to Oregon/Wisc. because we can. Why have more error in a project grid than is possible with LDP's?
Argument 2: The existing SPC zones meets required FGDC standards for allowable error in highway construction.. The worst case scenario in our state using a combined scale factor in the existing SPC zones is 1:8700. The best case is 1:zillion. 80% of the state is better than 1:10000. The fewer zones to deal with, the better.
 

From what I've seen, neither of those arguments really comes into play.

The primary reason to use a LDP is that it allows us to work over relatively large areas without using any scale factors.  We therefore completely eliminate the "Grid to Ground" problem and all the sundry issues it can create.  Yet we still have a georeferenced coordinate system, and can take advantage of all the benefits of such a system.

Although I agree that the fewer zones we have to deal with, the better.  That's why I hope more states and municipalities start to follow the lead of areas like Oregon.  Otherwise, we'll have individual surveyors creating a lot of LDPs that are only marginally different from each other, leading to its own form of chaos.  And of course, if we have a bunch of "official" LDPs in various areas, we don't have to worry so much about poorly-documented or missing metadata.

Loyal,

Based on comments over time I'm sure we are very close in our opinions. I was simply addressing what some might have percieved as a disagreement.

Richard,

In general terms I'll agree with the 'primary purpose' statement. At the same time every situation has unique nuances.

Here in the valley there is a ton of data (images, coordinate data-bases, etc.) tied to our GIS coordinate system. The basis of the system is not what I would have done, but it's fine for GIS/Assessor work. The distortions introduced by the projection are too great for boundary work. Our solution was to generate an LDP for each Township. This allowed data sharing without teaching every user how to re-project the numerous products available. At the same time we maintained accuracy and precision for control and boundary work.

Our methods reference a single bearing system, and produce distance distortions where the projections meet that are less than our ability to measure (with RTK). There has yet to be a question or issue arise through our use of these projections. I'm not sure where regulation would improve our product. At the end of the day we publish information that far exceeds standards (both written and local practice).

I can certainly see where metadata standards must be developed and codified. At the same time I don't see the chaos you mention. What problems are created using multiple (even proprietary) LDP's?

TTYL, Tom

I can see my previous post is being misunderstood. My point is that using SPC as is for road construction design without 'worrying about scale factors' is as good now as it was 80 years ago when the worse case grid to ground error is 1:8700. This point can be supported by the Geospatial Positioning Accuracy Standards PART 4: Standards for Architecture, Engineering, Construction (A/E/C) and Facility Management guidelines.

Local accuracy standards for survey control will vary with the type of construction. Commonly
specified and reported Orders of horizontal closure accuracy standards are shown in Table A-1.
Relative accuracy closure ratios for horizontal A/E/C surveys typically range from a minimum of
1:2,500 up to 1:20,000. Lower accuracies (1:2,500-1:5,000) are acceptable for earthwork, dredging,
embankment, beach fill, and levee alignment stakeout and grading, and some site plan, curb and
gutter, utility building foundation, sidewalk, and small roadway stakeout. Moderate accuracies
(1:5,000) are used in most pipeline, sewer, culvert, catch basin, and manhole stakeouts, and for
general residential building foundation and footing constructionmajor highway pavement,
bridges, and concrete runway stakeout work.,  Somewhat higher accuracies (1:10,000-1:20,000) are
used for aligning longer bridge spans, tunnels, and large commercial structures. For extensive
bridge or tunnel projects, 1:50,000 or even 1:100,000 relative accuracy alignment work may be
specified.