State Plane Coordinates Are Clearly Defined. Period!
That is what happens when you get people who know nothing about control work doing control work. If I told stories about all of the dumb stuff I have heard from other surveyors, yes, you would think they are dumb.
This all happened many years ago. But I am sure those drawings still exist, and I can see it happening again...they see a coordinate on a drawing, and try to use GPS (or OPUS) and all of a sudden they are missing by 100 feet. In fact, the last time I heard about it was when someone was trying to put a lat/long at the point where jets park next to a jetway (airport gate), so they got coordinates off of one of the old drawings. This was so the airplanes could check their inertial units prior to flying over the ocean (before all planes had GPS, which wasn't all that long ago, I think they still use inertial as a backup). Of course it wasn't correct, because they were taking what looked like a true SPC and converting it.
So for those of you that only use "true SPC" are you doing all your work in meters?
I get that there can be some problems with modified SPC's. I think that mainly comes down to lack of metadata.
I'm just wanting to learn if there is a better way I could be doing things. LDP's seem like a great option but would require even more metadata than modified SPC.
This is the metadata for the job I'm finishing now. I put it right under the North arrow on my plat. All (or in some cases several) prominent points are labeled with true State Plane Coordinates. In this way, the SPCS serves as a point addressing system. The bearings and distances are in my LDP grid. This same metadata is on my plat as well as my field note description (Texas is a metes and bounds State):
Bearings related to a local grid having an origin of North Latitude: 32°23' and West Longitude: 94°52'. To convert reported bearings to bearings related to Grid North for the Texas Coordinate System of 1983, North Central Zone, rotate reported bearings counter-clockwise by the mapping angle of 1°58'54".
Distances are expressed in US Survey Feet as measured horizontally along the surface of the Earth. To convert reported horizontal/surface distances to distances measured along the Grid for the Texas Coordinate System of 1983, North Central Zone, multiply reported distances by the project combined scale factor of 0.999928.
Cartesian Coordinates are expressed in US Survey Feet as determined by GPS observations made on site and are related to the Texas Coordinate System of 1983, North Central Zone, HARN - 1993 Adjustment.
> So for those of you that only use "true SPC" are you doing all your work in meters?
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> I get that there can be some problems with modified SPC's. I think that mainly comes down to lack of metadata.
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> I'm just wanting to learn if there is a better way I could be doing things. LDP's seem like a great option but would require even more metadata than modified SPC.
That's not a genuine statement. While yes, the Texas Coordinate System of 1983 is meters, the NGS, after being requested by the State of Texas, also publishes their data in U. S. Survey Feet.
I work on the grid, so I have to have the meta data, but if you try and mix and match, eventually, at least around here, you're gonna encounter a 20' problem.
LDP's are great and Shawn built a great one and it serves their purposes well. I travel so much over so many counties, it's just easier to keep it in it's native format. I cringe when I send out digital plats that I know I may have to stake off of when an engineer see's it in grid and I worry (and spend a helluva lot of time checking) to see if they molested my data up to surface.
Sorry if my metric comment came off as smart ass. I didn't intend it to.
I appreciate all the responses especially the LDP info.
So for those of you that only use "true SPC" are you doing all your work in meters?
Bite your tongue!!!!
We went through wayyyyy to much of that!!!
I thought meters would be no big deal-boy was I wrong.
I don't think any more projects will ever be done in metric-what a nightmare!!
Disclaimer: I am not a GPS "guru". I was brought up on surveying the now "old-fashioned" way (with a conventional instrument, not a chain and compass). I have not really had to deal with SPC's, scale factors, etc enough to be an expert, so please pardon my ignorance. But...
I am trying to learn this stuff as it is the present and future of land surveying. Here is what I do not understand -- I read a previous post by Shawn Billing where the scale factor was .999928. From what I understand, that would amount to 0.4' over a distance of 1 mile or approximately 1 tenth of an inch over 100' (less than 1/8" over 100'). I am wondering, what project would require a tolerance of something that tight? Or is it just simply nothing more than a number. I have always wondered why it was necessary to measure within a 0.01' just to drive a 2" diameter pipe in the ground other than because we can. I know in the academic world and on paper, these things are very important, but what about in the real world? Once again, don't stone me, I am merely trying to increase my understanding of the subject and apply it to "my" personal knowledge and experiences. I know for a fact that Civil Engineering is not an exact science. We have factors of safety and derived constants. Is surveying an exact science to 8 decimal places? My fear is that the younger generation of surveyors will know how to take coordinates out to 8 places, but will not have much of a clue how to survey.
>My fear is that the younger generation of surveyors will know how to take coordinates out to 8 places, but will not have much of a clue how to survey.
This is always a legitimate concern. But it is possible to survey with precision and honor the laws regarding land boundaries. It's silly to ask, but take it to the other extreme, can only surveyors with poor measuring methods produce proper surveys? Of course not. So I say, let's aim for both precision and proper surveys.
As to the 0.10 in a hundred feet being trivial. Hopefully any of us could find the monuments even if they are a foot off in a thousand. But Dad always taught to me to be vigilant about hundredths so our results can be tenths. If you pee away all of your error budget on something avoidable like combined factors, then all of the other random errors will likely only make your work less accurate.
"I work on the grid"
That must be nice - I'm usually at an elevation of ±7000', so I work on the ground and calculate on the grid (if required), its just too far to dig to get down there. I have few, if any, clients that have even the faintest comprehension of what they mean when they tell me "We have to have this on SPC", and I usually usually refuse. When they protest we have have an in-depth discussion, and more often than not they come to the realization that the SPC requirement comes from an item on some agency checklist, and has no value to them in the real world...
I am trying to learn this stuff as it is the present and future of land surveying. Here is what I do not understand -- I read a previous post by Shawn Billing where the scale factor was .999928.
What I think you see in this discussion are regional differences. .4' per mile is something I just never see. It's more like 1.6' per mile where I'm used to working and it can approach .8' in 1000' in some areas.
So in places like that you really don't want to build structures based on that kind of error. Let alone tie property lines and write descriptions.
Where I am it's always been the norm to work on the surface with a coordinate system able to simulate ground distances. In other areas SPC is often "good enough".
> "I work on the grid"
>
> That must be nice - I'm usually at an elevation of ±7000', so I work on the ground and calculate on the grid (if required), its just too far to dig to get down there. I have few, if any, clients that have even the faintest comprehension of what they mean when they tell me "We have to have this on SPC", and I usually usually refuse. When they protest we have have an in-depth discussion, and more often than not they come to the realization that the SPC requirement comes from an item on some agency checklist, and has no value to them in the real world...
🙂 :good: :good: :good:
> Sorry if my metric comment came off as smart ass. I didn't intend it to.
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> I appreciate all the responses especially the LDP info.
No worries. I kinda thought it was funny to be honest but tried to respond as professional as possible. I'm very much a smart a$$. 🙂
I prefer "modified" SPC over LDP simply because grid north is still grid north. The coordinates in most of my area are large enough to be about 1000 feet different from grid. I realize it is quite possible to use a LDP that north matches SPC grid north but how often is that actually done?
James
> "I work on the grid"
>
> That must be nice - I'm usually at an elevation of ±7000', so I work on the ground and calculate on the grid (if required), its just too far to dig to get down there. I have few, if any, clients that have even the faintest comprehension of what they mean when they tell me "We have to have this on SPC", and I usually usually refuse. When they protest we have have an in-depth discussion, and more often than not they come to the realization that the SPC requirement comes from an item on some agency checklist, and has no value to them in the real world...
LOL! That's funny Jim. I suppose, you're correct from a purist standpoint; however, I have projects at 200' to as much as 800'. Normally I'm working around 400' so the elevation factor really isn't as much of a problem as where I'm at in the zone. Working slap in the middle of the zone on one project begins to give me fits with CSF's in the range of 0.99984. But even then, we survey on the surface and Carlson has the routine to reduce to grid when reducing the traverse data at each point so it works VERY well and I maintain my unmolested coordinates. 🙂
Yup. The one and only that we were on, we actually went and bought a metric 25' tape because go back 0.532 the third time will only serve to piss me off. 🙂
I hear ya, but it's simply a number to reduce one to a plane that allows for much more synergy from a survey product than 5000x5000 does. Like with anything though, the more bells and whistles it has, the more you need to read the stereo instructions. 🙂
As far as where to put the corners, it's irrelevant unless you're a dope that runs everything out to call distance instead of actually evaluating evidence and applying proper construction principles.
Usually, the point of a LDP is to simulate ground distances and swing the bearings in the project area to better match true north. Much like in a State Plane system at the central meridian geodetic north=LDP north, then as you move east and west LDP bearings drift away from true north just like State Plane.
You can construct a LDP that matches state plane grid bearings but you need to set the origin at the same longitude as the state plane origin and then apply a scale factor to bring it up to surface-kind of defeats the purpose, you may as well just modify the state plane and subtract or add a number to the coordinates to get different numbers-in essence a modified SPC system.
State Plane Coordinates Are Clearly Defined. Period!
I see the same thing. What’s up with this? I find most engineers I deal with and some surveyors cringe when I recommend local project control. Most think that it is so easy to establish control from space now that why do you even need site control? Why not just list SPC or NAD83 or some such thing. That way it will last forever as proposed to on the ground control that gets destroyed before the project goes to construction.
> Usually, the point of a LDP is to simulate ground distances and swing the bearings in the project area to better match true north. Much like in a State Plane system at the central meridian geodetic north=LDP north, then as you move east and west LDP bearings drift away from true north just like State Plane.
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> You can construct a LDP that matches state plane grid bearings but you need to set the origin at the same longitude as the state plane origin and then apply a scale factor to bring it up to surface-kind of defeats the purpose, you may as well just modify the state plane and subtract or add a number to the coordinates to get different numbers-in essence a modified SPC system.
Actually once you get the bearings you want, you scale the system to the scale you want, then adjust the false N and E and you can get it to spit out the same coordinates as a modified system. the benefit is that the coordinates in the Scaled SPC LDP projection are all directly tied to geodetic positions. No need for localizations and the coordinates can be instantly transformed to any other system.
It's the same with the surface coordinates dot uses. The underlying geodetic numbers are the same as the SPC for each surface coordinate and there is no need to calibrate.