The most technically correct way to transform coordinates between any two projections is to go through latitude and longitude. So if you have a coordinate list in SPCS and you want to convert it to LDP, you would convert those SPCS coordinates into latitude and longitude and then convert the latitude and longitude to LDP. This method would be lossless.?ÿ
I should add that this assumes the underlying datum is the same between the SPCS and LDP. If they are different datums an extra step would be needed to transform from one datum to another:
SPCS(datum xx) > Latitude Longitude (datum xx) > Latitude Longitude (datum zz) > LDP (datum zz)
So if you have and SPCS, NAD83 coordinate list you would convert it to LL(NAD83) (mindful of adjustments and epochs). You would then convert that LL(NAD83) to LL(__TRF2022) (again mindful of adjustments and epochs), and then convert the LL(__TRF2022) to SPCS2022/LDP2022.
?ÿ
Comment - the convergence angles in those 5 existing Texas zones have got to be huge in the east and west ends of your state, leading people to adopt local rotations.?ÿ Add to that the 4000' ASL elevation.?ÿ I just picked out an El Paso area datasheet at random - Convergence over -3?ø, CSF distortion 1:5,000.
Holy 1936, batman! I better stock more chain menders in my utility belt!
With LDP zones covering several Texas Counties it is very likely that any one surveyor would spend 99% of his career in just one zone.?ÿ People must look at all the problems using the current system causes and assume that a new system would be still more difficult. The truth is just the opposite.
?ÿ
?ÿ
?ÿ
?ÿ
I was unaware that recap had any coordinate system capabilities (it will do units). For that matter, I'm don't think any of my point cloud software has that ability either.
Hopefully you are truncating after you scale.
As for TBC and lambert LDPs I'm pretty sure they can at least be imported. Oregon's list LDPs include several of that flavor and ODOT provides a Trimble Coordinate System Manager file (CSD) for all 39 zones at https://www.oregon.gov/odot/ETA/Documents_Geometronics/current_ocrs.csd.
I've now completed examining 101 recently recorded and consecutively numbered, surveys in Multnomah County. It seems that my initial shooting-from-the-lip guess was pretty close. Of the 101, 73 are local coordinate systems, 21 are on state plane (all but 2 scaled to ground), and 7 are on the LDP system.?ÿ?ÿ
21 are on state plane (all but 2 scaled to ground)
How do you know what coordinate system was used? Does the state or county require coordinates? Here I have yet to see coordinates on the relatively few I have looked at, and usually not enough metadata (statement, scale, BOB) to say. One survey of interest near family land that I checked I'm fairly sure gives SPC distance and bearing, but it doesn't specify.
What are the odds the other 2 are scaled but don't give the factor?
Actually, I'm impressed if 19 of them did specify.
there is a Basis of Bearings statement on every one.
I have mostly seen "east line if section assumed to bear x xx xx" and it's up to the reader to figure out if that was to match an older survey, a SPC bearing, or something else.
Oregon law requires a fairly comprehensive narrative along with a definitive basis of bearing.
Oregon law dictates that there be a Basis of Bearings Statement and that the County Surveyor review survey maps submitted for filing. The Portland area County Surveyors take their responsibilities seriously and over the years have pretty much beaten us all into submission - the basis of bearings statement being a favorite thing to comment on. I'll note that the current group have relaxed on that some, thankfully. But the die has been cast.
I'd like to further note that our situation with GPS is different from that part of the county which lies east of the Cascade Mountains. There are just too many trees to use RTK routinely. GPS only RTK was practically unusable. With GLONASS added you can use it enough to get your project on a grid system. But there are still a lot of outfits around that do not have GPS, nor any idea how to use it. This is in contrast to my experience in Oklahoma, where everybody had GPS, but many had no idea how to use it.
No I never truncate, well,,,,not since the 70's early 80's.
Since then that isn't something that's done. You lose the connection to state plane.
As far as importing an off brand LDP into TBC, maybe. That isn't the point. My point is that if there is someone sitting in an office in Texas that needs a SPC drawing file in ArcView or AutoCad then it's simple to give that to them when a DAF has been applied to the work. If the LDP is a canned system accepted by all the software sitting in the pull down lists, then I will gladly use it, otherwise not interested, I will continue to scale up SPC like we have done since the 1960s.
As far as someone getting my scaled coordinates that only happens when requested and then it's not my call anyway, the client determines the metadata at that point, otherwise those are only in-house numbers to simulate ground distances for the survey.
Since then that isn't something that's done. You lose the connection to state plane.
Proper metadata provides that connection just as well with truncation as without.
From my experience, the metadata never makes it out alive. I have spent a large portion of my professional career working with others data. For the most part, it comes sans metadata. I'm not saying that the original surveyor does not provide it to their client, but by the time it makes it through the engineer and back to me it is gone. There is nothing worse than trying to figure out if the coordinates that look suspiciously like SPCs are in fact SPCs or how they were scaled. If it is not SPC then it is better that it does not look like one. Once you scale an SPC it is no longer an SPC.
Friends don't let friends scale without truncating.
No way I would truncate, but the shift is too large to ever think it's state plane. Who would think that?
I've also never ever seen a state plane construction job. Not one that I can think of, in fact nothing is surveyed state plane with very rare exceptions, so the assumption when getting coordinates is that it is not state plane.
But who are these people getting my coordinates anyway, DOT demands a DAF for each project, beyond that coal mines or an engineering company might request it. Otherwise mappers, regulators, GISers, those people want canned projection data for their catalog info and having a one step conversion makes my life way easier.
I've also never ever seen a state plane construction job.
You just have to live in the right place.?ÿ Around here the combined factor runs about 0.99996, so all general construction can be done on state plane.
Yeah, I hear that, my least DAF is 1.0002 or a CF of .9998 or 200PPM. From there it only gets larger or smaller depending how you look at the number.?ÿ
Central Montana at low elevation can see a CF of .9992 or .8'/1000'. The last job I did strictly SPC was for a railroad right of way, they wanted a 200' wide right of way and didn't first understand why my plat showed 199.85'. I had to tell them it measures 200' on the ground with a tape. They insisted on SPC.
The areas didn't match the dimensions and you can't just multiply areas by the CF to get the correct number.
Made the reviewers exceptionally happy.?ÿ
At .99996 you basically are working in an LDP. There should be little need to use something else.?ÿ
?ÿ
@john-putnam I was there with John, in hindsight I wished we had NOT created the single parallel Lambert zones, they continue to be slow to implementation in software packages 10 years later. We have a few Oblique Mercator zones and oddly those are supported in everything I have tried them with.
I think the single parallel Lambert zones could of been done with a TM projection, although it would of likely created more zones to keep the distortion at the design goal.
@norman-oklahoma other than ODOT projects I see the same thing, one odd thing is all the firms still scaling SPC to ground, my gosh, if you want ground why not just use the appropriate LDP that is in place?
SHG
It is really disappointing. I figure that ignorance is the only possible explanation. Maybe it needs to be re-addressed at a future conference.
You're probably aware, but the next State Plane will use single-parallel Lambert instead of two-parallel Lambert, so any software developers wanting to keep current with 2022 will need to have that projection type in their database.
