Can anyone explain to this ancient surveyor the necessity of constantly changing datums? ???? TIA
Tectonic plate motion, local motion, improved modelling of the earth, more precise observations of both passive and active marks, improved projection parameters, software and hardware improvements, to name a few.
We also kept kicking the can down the road when it came to adopting the global reference frame as our geodetic basis, which honestly has caused a lot of confusion and no shortage of screwed up datasets.
In other words the new datum is more closely tied to satellite orbits removing all the calculations needed to transform GNSS observations to biased and disappearing ground control.
Exactly. I for one am happy that we're finally going to have a global basis, and active control defined in 4D.
It probably doesn't matter which datum you use to lay out house lots.?ÿ But consistency in the larger picture is important. The average surveyor now has measurement capability that is better than the self-consistency of the present datums.
Thank y'all, as B93 mentioned house layouts are simple (30 years worth) but I never considered the geodetic aspect. Thanks again! ?????ÿ
Still alive, still learning and love it.
Just saw what you described for my first time recently.?ÿThe site had control set already, and I suppose the engineer??s design plans were meant to convey that control and the associated coordinate system provided by whoever flew the LiDAR, but the engineers brought the LiDAR into Autocad, and Autocad appears to have a bug(?) where a surface defined with a *.tif is brought into a position that is shifted from it??s native location (this one was shifted ~4?? South and ~11?? West). Needless to say the engineer??s plans did not include any control. ?ÿRegarding the shifted surface, there are threads that explain this is remedied by shifting the surface a half cell-width back but I don??t think that??s correct and prefer to extract the extent of the tif in QGIS (carefully maintaining the same projection settings in QGIS and Autocad), then import that extent into Autocad and shift the surface at the lower left corner to match the extent from QGIS. Anyway, all that definitely made staking their design interesting.
the engineers brought the LiDAR into Autocad, and Autocad appears to have a bug(?) where a surface defined with a *.tif is brought into a position that is shifted from it??s native location (this one was shifted ~4?? South and ~11?? West). Needless to say the engineer??s plans did not include any control.
I don't think I have ever encountered that bug. Never had problems creating surfaces from DEM and TIF files.
But even if it is shifting it by a couple of feet, it's an existing grade DEM with a cell size of several feet or more. If it's being used for anything other than existing grade for very rough, preliminary design, or simple contour lines, there are bigger problems than an import glitch.
Can anyone explain to this ancient surveyor the necessity of constantly changing datums?
The 2022 is the first new datum since 1983. All those other iterations were new "realizations", not new datums.?ÿ?ÿ
Basically, on the way to creating a really skookum new datum in the 70's and early 1980's, GPS happened. The basic assumptions of NAD83 were developed without the benefit of GPS (and later GNSS) measurements. The NGS has been playing from behind ever since.?ÿ Future generations of surveyors will know the 1927 datum based on terrestrial measurements, the 2022 datum based on satellite measurements, and the bastardized piece of crap in between.?ÿ?ÿ?ÿ
piece of crap in between
I agree with your facts but think your evaluation is a bit harsh, as I think NAD83(x) have always been more self-consistent than 27.
Each realization has been closer to what GNSS gives, but the spacing of stations having both classical and GPS data is large.
I believe the closest station to me that is held in those recent realizations is 40 miles away (several tri stations are gone, one has not been officially GPS measured, and one is under a tree in someone's front yard). I find approximately 1 ft difference with GPS on those few local marks I can compare, a tri station, rm's of the tri station under the tree, and an intersection station. That demonstrates phenomenal accuracy in the old theodolite observations, but due to the distance pretty coarse compared to what can be done now.
A one parallel Lambert means that from the central parallel north and south the grid scale will be larger than one. Often the ellipsoid surface is above sea level, sometimes quite a bit. This means that in costal areas the grid distances will be longer than ground distances. By a considerable amount near the edges of the zones. Of course it work better as elevations increase, but care must be taken to design the zones.?ÿ
Cayucos, California is a magic spot on the California central coast where the CF is so close to 1 that grid is the same as ground for all practical purposes.
But I??m just a simple country surveyor, Jim.
Lambert projections are sometimes nice for that, not only can you be at the edge of a zone with a close to 1 GF but you can be near the central meridian so convergence is minimal. With Tranverse Mercator along the central meridian the GF is large when you get to the edge of the zone the GF approaches 1 but the convergence is large.
