Just messing around looking at city coordinate systems, I found this from Aurora, CO:?ÿ Microsoft Word - City of Aurora Requirements for Survey Control Drawings March 2019.docx (auroragov.org).
The thing that jumped out at me was this:
?ÿ 7. If State Plane Coordinates NAD 83/92 are used and converted to Project Coordinates (Ground)
include a note stating the Grid Factor (sea level factor combined with scale factor). Show the Grid
Factor to nine decimal places. If Project Coordinates (Ground) were truncated, note what number
was subtracted from the north and east coordinates. Project coordinates are to be shown to four
decimal places. (Emphasis added)
Now the sea level factor was used with NAD27 because the geoid and ellipsoid were considered coincident at Meades Ranch. That is not true for NAD83.
In essence, sea level factor uses orthometric height and elevation factor uses ellipsoid height. The difference between the two under NAD83 is usually around 30 meters, but it can be more than 50 meters. Taking the statement at face value, a surveyor would use the NAVD 88 height to approximate height above sea level instead of the ellipsoid height.
The term "combined with" is also vague as there are many ways to "combine" two numbers. Is is really too hard to say "multiplied by"? It's almost as bad as saying that the published coordinates are based on the Colorado State Plane Coordinate System with nothing further.
For me, as I struggle to learn the mathematics that supported surveying, the inconsistency in terminology from publication to publication made the job harder. Perhaps that's not true for surveyors learning on the job.
This piece makes it clearer than I ever could:?ÿ Elevation Factor | GEOG 862: GPS and GNSS for Geospatial Professionals (psu.edu)
?ÿ
Speaking of terminology, mathematically, is "truncated" the correct word?
Both 5.3 & 5.8 truncated to ones is 5.
But as used in the requirements is something like "subtract 100,000 from the northings"
So, North 100,005.8 would become 5.8.
The idea is to remove the "characteristic" portion of the number, not the "mantissa".
"Truncate" doesn't seem like the correct term.
?ÿ
I assume they meant that if all northings fall between 7654328.123456 and 765591.243567 that you could drop the 765 so as to have smaller numbers, especially if the eastings provided an identical opportunity of dropping the first thee digits.?ÿ The results should be identical.
Truncated usually refers to dropping least digits. To be clear this example should specify truncate high order digits.
But the right way I think is to say reduced coordinates where some round number is subtracted.
Here's a piece of insight as to why some DOTs don't do that. I had no idea about software considerations for coordinates.
http://onlinemanuals.txdot.gov/txdotmanuals/ess/geodetic_surveying.htm#i1028861
It??s been 20 years, so I don??t remember the details, but I was a cad tech on a large TxDOT project that turned into some kind of perfect storm of coordinate problems. First, a team of consultants, for reasons I??m still not sure I understand, decided to move an SPC zone line about 25 miles north. If I recall correctly, it had something to do with minimizing bearing discrepancies (basically they found a nice, long tangent in the design where the artificial zone change netted a bearing really close to the real bearing from the otherwise ??natural? zone. So, like, the back stationing for that one tangent differed from the forward stationing by sub-minute values, or thereabouts. So that, in and of itself, caused some obvious issues. Nothing horrendous, but still required diligence. The other issue was all cad was to be output in micro station, and all descriptions and area calculations to be the same. However, when we sent each parcel description to TxDOT for review, they were obligated/instructed to use LDD to run map checks on our work. Well, turns out microstaion (at the time) calc??ed off some obscene number of significant digits, but auto cad didn??t. We went around for months with reviewers who insisted our areas were all wrong and that our tracts didn??t close to 8 decimal places. I spent weeks, at one point, doing nothing but running mapchecks of drawings and legals for identical parcels in both software, and maybe 2 of them came out with close enough matching data to pass muster. I think after that exercise the lightbulb finally went on for somebody up high, because I just remember we suddenly were getting back significantly cleaner redlines on our work.
I didn't read the manual since I've read others. I would pay attention to them, they lived the experience for decades. From the era of large traverses covering hundreds of miles to the GPS/computer era. I've never had one of my DOT partners engineers/surveyors have issues with the process. One engineer who raised a stink about not getting DOT work did a project using International feet when SPC is defined as US feet. That caused a momentary problem until we figured it out. He didn't get anymore /DOT work. But, of course that's a separate issue from Datum Adjustment Factors.?ÿ
Very well said. My problem with the Aurora document was the use of "sea level factor" in conjunction with NAD83. The best way I can fit those two together is to use NAVD88 height instead of ellipsoid height. NAVD88 is close to height above sea level, but using it to compute a combined factor in NAD83 is wrong.
It's easy to see how the term got carried forward from NAD27. The sea level factor and the elevation factor perform analogous operations in their proper contexts, but they're not the same thing. It may be ok for seasoned veterans to use the terms interchangeably, but new people need clarity in order to understand.
As I said, I'm too picky and I've been caught using outmoded and confusing terms in teaching mathematics. For example, in calculus I had to concentrate on saying "the area between the x-axis and the curve" instead of saying "the area under the curve." I was taught the looser term; graphing software has made the tighter term mandatory.
Probably made too much of it, but imprecise wording is the source of many misunderstandings.