toeknee:
TLDR: Turning off SBAS should disable DGPS. You’re on the right track by setting up a local RTK base. Consider hiring a surveyor before building.
- There are a plethora of GPS solution types. They are reported in the 6th data field in the GGA statement
From http://lefebure.com/articles/nmea-gga/
Fix type. This is always a single number. Reportable solutions include:
0 = Invalid, no position available.
1 = Autonomous GPS fix, no correction data used.
2 = DGPS fix, using a local DGPS base station or correction service such as WAAS or EGNOS.
3 = PPS fix, I’ve never seen this used.
4 = RTK fix, high accuracy Real Time Kinematic.
5 = RTK Float, better than DGPS, but not quite RTK.
6 = Estimated fix (dead reckoning).
7 = Manual input mode.
8 = Simulation mode.
9 = WAAS fix (not NMEA standard, but NovAtel receivers report this instead of a 2).
Autonomous is the most basic solution type.
RTK Fixed is is usually the most accurate and precise.
Note that list isn't totally in order of accuracy.
DGPS requires a differential correction source (hence the "D"), generally a Space-Based Augmentation Service (SBAS) such as, in the US, WAAS.
If I'm out running my Facet rover in RTK and I lose the RTCM link, it will drop down from RTK Fix to DGPS in 60 seconds. If I'm down in a valley and my Facet doesn't have good sky vis to the south (I'm in the US and the WAAS satellites are geostationary--that is, over the equator) I won't even get a DGPS fix and the Facet will report an Autonomous solution.
2. <B>**You can turn off DGPS by disabling SBAS**</B> (which includes WAAS) satellites. Some surveyors running RTK explicitly turn off SBAS. Here's why:
<LIST type="lower-alpha"><LI>If you get a DGPS solution using the WAAS differential corrections, the receiver's reported solution (coordinates) will be in ITRF, or so I've come to believe. I haven't found a authoritative source for this, but that's what I've gathered from surveyors posting on one of their most active web forums. You can web search (this is Google syntax) "waas datum site:surveyorconnect.com" to read what I've read.</LI>
<LI>- US CORS stations provide RTCM messages based on NAD83. If you're running network RTK utilizing a US CORS station, the rover wil output NAD83 coordinates.</LI>
<LI>- Likewise, if you setup your own base and configure the base with fixed coordinates in NAD83 or ITRF, your rover's output coordinates will be in either, respectively, NAD83 or IRTF.</LI>
<LI>- Some surveyors running RTK in NAD83 will TURN OFF SBAS so their receiver's solution (output coordinates) doesn't jump to ITRS and then jump back to NAD83 when they briefly lose and then reestablish the RTCM feed. If your're working at the limits of your RTCM comm link, this gets very old very fast. NAD83 vs. ITRF are about 5.7 feet different in my part of the US, and that's significant.</LI>
</LIST>
3. <B>**Standard errors**</B> provided in marketing literature for just about every GNSS receiver sold <B>**are well-known to be very optimistic**</B>. Ditto for standard errors reported by the devices while in operation. It's just the market; like horsepower ratings on vacuum cleaners and cold-weather temperature ratings on sleeping bags.
Some of the reason for the optimism is that the receiver calculates a standard error based on what it knows, but there are error sources that it might not know about or that it doesn't model accurately. It's doing what it can, just take it with a grain of salt.
On top of the optimism, the standard errors are, unsurprisingly, generally reported like standard deviations, 1-sigma, expressing a 68% confidence interval. So 32% of the readings will fall outside of the +/- 1-sigma range. And 1% of the readings could likely be terribly off. So to get a 95% confidence interval, you need to approximately double the standard error. And then add in the optimism factor.
So if the Facet is reporting 3cm, that can be interpreted as 68% of the readings will fall +/- 3cm of the actual coordinates. So 68% of the readings will fall in a 6cm range (a 3cm diameter circle). ~95% of the readings will fall withing 12cm. And those are optimistic estimates, so add in (or multiply in) a factor to account for the optimism.
I do a bit of post-processing and least squares processing of GNSS data. Due to the well-known optimism factor, a few widely-used least squares adjustment softwares include a configuration setting to account for the optimism. The s/w I use takes the setting I input, raises 2 to that setting, and then multiplies the reported error from the GNSS receiver by that number. The take-away is that the reported errors are so optimistic, widely-used post-processing software packages provide an <B>**exponential scale**</B> to convert the optimistically-reported errors from the GNSS receivers to more realistic errors.
4. <B> **Municipal GISs are not known for their accuracy.** </B>They generally exist as land tax maps, 911 systems, code enforcement, etc. (Yes there are municipalities that fly drones periodically and look for unpermitted swimming pools!) The vast majority of municipal GISs are not accurate enough for surveying property boundaries. I doubt any surveyor would use GIS coordinates for anything more than a rough starting point to search for evidence in the field.
5. Setting up a local RTK base is the way to go. The short baseline from base to rover will help a lot. By far the limiting factor on the a Facet RTK Base-Rover setup is the RTCM radio link. If you have cell/internet availability over your property you have an alternate to the radio link. Run a couple static sessions for the base on different days, get multiple post-processing solutions, compare the results.
6. If you think you've found the corner monuments, you can always run static on each corner and post-process that data with OPUS or CSRS-PPP, etc.
7. <B> **Consider hiring a surveyor.** </B> Besides having the equipment and experience to run a straight line between two corners, they also have the knowledge and expertise to check that the monuments in the ground are actually your property corners, and not something else. They know how to interpret deeds (that's a complex topic all by itself, and the numbers in the deed descriptions often appear "wrong" if you think like an engineer; land surveyors know how to sort things out. ) There are three different legally-recognized definitions of a "foot" in the areas I work in; they are pretty close but I gather your fence is going to be thousands of feet long, so minor differences matter. The good ones have insurance in the event things get built in the wrong places. They will generally also survey enough of the parcels adjoining your parcel to check that things are in the correct place for you and for your neighbors. Property lines and corners aren't yours alone, they are shared with the neighbors, and it's important that no one gets shorted. Finally, if your neighbor is unhappy with the outcome, the first thing you might say is "I hired a surveyor, why don't you hire your own surveyor and we'll let them sort it out." It's way easier then hiring attorneys and sitting in court rooms.
That was a lot. I really appreciate that too - I generally write a lot myself (irreducible complexity cannot be overcome), and I appreciate it when others do the same. At present I don’t think I can reply to everything - I was out all night running a PPP survey for my base, but I would like to reply to the last item (and maybe one other), specifically “consider hiring a surveyor.” The short answer to that is that I have considered it and investigated it and what they’re asking is a lot more than I want to pay for what they would give me under my present circumstances:
My land does have private “neighbors” (meaning private land bordering it), but there are absolutely no fences or building sites or activity of any kind in the area at all in the entire section. In 1975 half the section my land is in was subdivided into a tract - and no one bought any of the pieces and now they’re all being held by a holding company that doesn’t have any contact information and is registered to a non-descript office that is closed 24/7 and only has a PO Box that doesn’t accept mail (they auto-return mail to senders). Perhaps someday they will try to develop it, but given where it is, and the market of the area, that isn’t likely to happen in my lifetime. The closest standing structure to my property is over 2 miles away, and the closest paved road is over three miles away. There are legally recognized (by the county) dirt roads providing access to my property, but there are no legally recognized easements through my property (or any adjacent piece) to the isolated pieces behind my land, so anyone that ever does try to become my neighbor (I’m trying to buy as much of it as possible because I don’t want neighbors complaining about me taking off and landing my planes) is gonna have a heck of time even getting legal access to their property along 3/6 of my sides. Additionally 1/6 of my sides borders BLM land, and they aren’t known to squabble (in my area) over a fraction of an acre. That leaves two sides that are of moderate future concern (though again I am trying to buy those pieces if I can find a way to contact the owners). The isolation of my land from real neighbors is a significant part of why I bought it.
Now that said, the legal description of my land was as you say a bit of mess at first to my mechanical engineer mind, but I do know exactly how to interpret it now. My land is legally exactly 3/4 of the SW corner of the SW corner of the section I am in, so 30 acres, with a perimeter of a mile (give or take a few feet). Additionally, the monuments I am referencing to are the USGLO survey monuments - the same ones that were referenced to for the entirety of the 1975 tract I mentioned above. None of them are directly on my property (because the nominally ten acre SW corner of my SW corner of the SW corner of the section isn’t yet mine), but I could absolutely dig out some stakes and mason line and a 100 ft tape and chain my way to my corners just like old fashioned surveyors used to do. Using the GPS is just so much more convenient and will let me do stuff like positioning things within my property more easily, differential elevation surveys, and keeping track of my roads, runway(s), piping, and wire runs etc.
In light of all of that, I don’t really see what advantages a proper insured survey affords me. Neighbor concerns are pretty much moot, and if I ever do have neighbors at my fence lines (I won’t fence all the pieces, just the core piece) that complain about my fence placement and they can prove I am wrong, I won’t fight it, I will just move the fence.
This whole thing is, and is expected to continue to be, a learning experience. I am not afraid of that, and I am not afraid to get my hands dirty or do hard work to avoid paying someone else to do it for me. Thank you for the input you provided and all the time that you took to do that. I really appreciate it.