Expecting the North Arrow to always be pointed exactly North will is dangerous.
You need to get out there and locate the physical evidence and actually retrace the survey; not just plug a bearing into your GPS and start blazing a line.
We suspect a Forester's technician used a handheld GPS through the Forest. He's only 200' off in places, not too bad.
> I can't see a magnetic bearing basis as being all that useful to a future surveyor who is surveying an adjoining tract and wants to locate the boundary with certainty and as little fuss and muss as possible.
Close enough to trip over the old barb wire.
True North
With RTK gps can be nearly as bad as ASSUMED or Magnetic North.
Because the Base station could be 3 miles East, or 4 Miles north. Around here, it is about 0°00'56" per EW mile. But, you can generally assume that it is within a mile. So, it sort of helps, but often 1 minute of arc can be the difference between good and bad. So, I say it again. GRID north is the way to go.
OR a reference to Grid.
BOB = the line between the N1/4 cor, and the C1/4 as per ___ plat, recorded in Bk___ Page___, and for GRID north, rotate these brgs CCW 01°26'47". Now you have tied the brg system down.
N
> Even state plane coordinates are monument dependent.
Well, in the sense that the CORS sites are monuments, that's true, I suppose, but the idea is that GRID NORTH is not monument dependent in any significant way. You can quite easily calculate grid North from geodetic North determined by astro observations and, with a little care, you should find that your determination will match what might be done with GPS.
Question about True North Surveys?
> To those of you that reference your surveys to true (or astronomic) north. Do you adjust all of your bearings in the survey to account for meridian convergence (i.e. true geodetic inverses throughout)or is it only true at one meridian on your survey?
As a rule, I wouldn't use true/geodetic North as the bearing basis of any survey with an East-West extent that had a change of convergence of much more than a few seconds. There are plenty of urban parcels where the convergence across the parcel is less than that, so the effect of convergence is negligible.
Naturally, better practice is to calculate grid North from true/geodetic North determined by astro observations since it's so much more useful when many surveys are to be pieced together over an area where convergence is significant.
Question about True North Surveys?
GLO/BLM PLSS Surveys have been returned using True North Bearings for about two hundred years. As anybody who has ever tried to “close” a Section using True Bearings knows, it DON'T work using standard cogo routines.
I have returned a few (VERY FEW) Records of Survey expressed in True Bearings, but I certainly don't make a practice of it.
Loyal
BLM True North Surveys?
Whenever you close a section on the BLM survey plat, you have to add a convergency factor for the latitude that you are in.
I am sure that most know this.
BLM True North Surveys?
Not only that, BUT a they have already rounded the brgs, to the closest minute, most of the time. And, around here it is about 55" per mile, for EW convergence.
So, do you convert them to GRID first?
I mean, they degraded their work, with rounding.
That aughta be changed.
N
BLM True North Surveys?
Not really, if the monuments are out there.
I think convergence is about 2' per mile in Northern California.
Question about True North Surveys?
Rare indeed do most surveys encompass more than a section. Most of those a combination of private parcels and subdivisions and commercial uses. The bearings in the deeds came before the subdivisions. I sort these things out after recovering the physical evidence. Mons found in deeds or on record maps and plats. I take the best monumented line relating to my deed and I put that basis on it. I assume the deed means what it says.
If the adjoing parcel has a different base, just rotate it onto my base and note the adjoing data along with mine.
I do star shots in the rare occasions that a call is for due north. 10 minutes to observe the predicted elongation. If that fits the mons, I'm done with the survey on the ground. Where the fence is will be shown.
I use GPS to map backs of curbs throughout whole 1/4 sections to find how all the subdivisions fit, because so many record mons have been destroyed. Rebuild half a mile of an arterial street, and not one mon or whitness mark remains within the project. And the blocks several deep have had all the witness X's or nails replaced for wheelchair ramps.
If you are clear about your basis on your record, no surveyor will be fooled into not knowing how to relate it to his basis.
Stay away from any standard that wants to make all surveys follow one basis. It would be rediculous.
State plane all the way for highways and pipelines and such. But for local surveys, local mons related to local deeds is the basis.
Question about True North Surveys?
> State plane all the way for highways and pipelines and such. But for local surveys, local mons related to local deeds is the basis.
But if I understood what you just posted, you described projects where all of the local monuments had been destroyed and you were using curbs on the theory that there was nothing else to work with.
Having surveys with bearings such as accurate grid bearings that don't depend upon the existence of those monuments to figure out which way a line is running would seem to be a no-brainer. Is the real problem that title companies would freak out if a line that had been reported as "North" for a century suddenly was revealed not to actually run exactly North?
Question about True North Surveys?
I am afraid Kent raises a valid point....
I can say that sure, use the local BOB, if you like, BUT just tell how to rotate it to grid. Now, we have in immensely more traceable survey. I personally am using GRID for next to everything. And, letting OTHERS do that non objective thing. Sure, it may have a long history... but if it is not very traceable, who cares? I got into a mess last month, where the CITY had destroyed 90% of the NEEDED monuments in an old subdivision. It was a 1923 Subdivision. Since NONE of the old mons were there, and getting on the old BOB was IFFY at best, well, we are starting fresh, and it will be GRID. I am doing them all a big favor.
That's life!
N
i'm the only surveyor in my state that has shown an azimuth on a plan derived from solar observation. i have used venus several times to check a grid bearing during daytime, and brought some wonderment to an assistants day when i pointed the total station to the sky, told him to look through the eyepiece and said venus crossing in 5, 4, 3, 2, 1, now.
i'm all for astro obs to be included on plans. i retraced a couple of survey from well over half a century ago and they had been aligned to mag. north, but had included a table of the results of some star obs. well after applying a little grid convergance to get to our state grid system i found we were now within a minute of azimuth.
pretty freakin' neat when the age old survey is already aligned for you and you can now begin to figure out how it slots in with what's around it, without a rotation.
> I hope that the surveyors who are continuing the practice of just creating their own private "North" directions from the rumored bearings between pairs of perishable monuments on the theory that modern life won't destroy the monuments rethink this in the new year, particularly the GPS users who have the means at hand to do much better for essentially zero additional cost.
Will do. I've been wanting to habitually orient my surveys to Grid North via astronomic observations. With the ushering in of a new year I resolve to make this a reality.
I see more value in this practice in the urban and developed areas than in rural and forested property where obliteration of survey marks is less likely to occur.
Thanks for the professional suggestion.
Happy New Year!
> I've been wanting to habitually orient my surveys to Grid North via astronomic observations. With the ushering in of a new year I resolve to make this a reality.
I think you'll be surprised to find that your grid bearings via astro observations will in many cases be more accurate than one of the contemporary RTK efforts that shot and missed. It's mainly a matter of refinement of technique and I'm certain that you're up to that.
In the old days, we typically had to scale positions off a quad sheet to get the latitude and longitude of the station where the astro azimuth was observed to use in calculations. With handheld GPS, of course, even that isn't an issue.
> In the old days, we typically had to scale positions off a quad sheet to get the latitude and longitude of the station where the astro azimuth was observed to use in calculations. With handheld GPS, of course, even that isn't an issue.
Even more practical and/or feasible is to locate your cursor on Google Earth, I would think, to come up with a latitude and longitude of the observation.
IT IS IMPORTANT TO PRESERVE THE RECORD
Survey in whatever system you want, but the client hires you to serve his parcel. It is important that he knows it is substantially as it has been. He can see and comprehend a few hundredths or a few seconds, but gross changes make it appear that his parcel has substantially changed.
An honest survey on an assumed bearing is far more easy to understand than an alledged SPC survey, where you cannot quite figure out what was held, is it really grid or ground and where the SPC came from.
A few years ago I worked on a preservation survey, well GPSed, setting 2 required concrete monuments, with record SPCs. That parcel crossed the road but the Township was only acquiring the westerly side. This year we returned to survey the adjacent parcel to the South, both sides of the road. As we reconn we find very recent traverse points and find the Township had engaged a different surveyor to do the easterly portion of the prior tract. We are reconning up oue most easterly line to where we intersect a third common line with the newly surveyed parcel to the North finding a pipe then turn and follow that line up a hill to a common corner to that parcel and our easterly adjoiner. No visible evidence but I have a sound, 1' down I find a leaning pipe, not recovered in years. We ribbon it up so whoever comes back to their survey can in no way miss it. We continue South on the adjoiner and find a standing pipe, not ribboned.
We started from our previous traverse, locating a new set concrete monument with SPC on a common corner to the other parcel and our new survey. From a draft copy of the other new survey we disagree with the SPC, which is alledgely from our prior monuments just up the road. Our survey agrees angles and distance with 3 lines of new survey and we just ignore the bearings and SPC as not worth additional effort. We expected but did not get comment on the SPC difference.
The intent of the SPC system, (1:10,000 maybe) is to facilitate in locating monuments, and was sufficient for that purpose. To utilize same to relocate a missing monument requires subsatantial corroborating evidence. In my opinion more than 50% of missing monuments are not in fact missing, surveyors are using insufficient evidence and looking where they are not. What can only be said with certaintly is that they were not found in the wrong place.
Paul in PA
I get time from the handheld GPS too. It's kind of a one stop shop for information, other than the ephemeris.
Google Earth Or Statewide GIS Is Sufficient
to get you Latitude/Longitude for SPC convergence, scale and elevation for a scale factor. It can be relied on to start a search.
GPS during the survey allows one to converge the survey and that starting data.
Just some of the tools in one's toolbox.
You can go to Chevy dealer or AutoZone for car parts, but neither installs them for free.
Paul in PA
