How to Mean a BS-FS-FS-BS Angle Set
Angle Set Report
Occ Pt: 3
Ang Rt BS H Dist FS H Dist
Avg 94°53'30" 258.050 186.317
Hi 94°53'32" 258.050 186.317
Low 94°53'28" 258.050 186.317
Diff 0°00'04" 0.000 0.000
Obs Point H Circle Z Angle S Dist H Dist V Dist BS Circle HA Res VA Res SD Res HD Res VD Res HR Prism Type PC
BSD 1 0°00'00" 89°58'11" 258.050 258.050 0.136 0°00'00" 0°00'00" -0°00'03" 0.000 0.000 0.000 0.000 Prism -30 Offset
FSD 109 94°53'28" 90°40'31" 186.330 186.317 -2.196 0°00'00" 0°00'02" 0°00'01" 0.000 0.000 0.000 0.000 Prism -30 Offset
FSR 109 274°53'33" 269°19'27" 186.330 186.317 -2.198 0°00'00" -0°00'02" -0°00'01" 0.000 0.000 0.000 0.000 Prism -30 Offset
BSR 1 180°00'01" 270°01'55" 258.050 258.050 0.144 0°00'00" 0°00'00" 0°00'03" 0.000 0.000 0.000 0.000 Prism -30 Offset
The above is part of a MAGNET Field report. The original format has been corrupted. I'm embarrassed to say that I forgot how to mean a BS-FS-FS-BS angle set. I remember how to check the set to see if it meets our <= 5" requirement. I take the difference between BSD 1 and BSR 1, which is 1". Then I take the difference between FSD and FSR, which is 5". Finally, I add 1" and 5", which is 6", divided by 2 equals 3", which is <= 5".
Field Dog, you would mean in the following way in a field book.
direction ( mean D&R) direction angle
D 0-0-0 00.5 0.0 94-53-30
D 94-53-28 30.5 30.0
Note; there is no space between the D&R for the backsight nor the foresight. You mean the BSD and the BSR, this gives you 0.5 seconds. The next column is the direction and for the BS you want 0.0 seconds(you have a +0.5 mean and you want to make it zero). the next column is the angle. It is the angle of the mean of the FS D&R minus the mean of the BS mean or 94-53-30.5 minus 0-0-0.5 = 94-53-30.0 This is your mean angle for position 1.
Some record books are a little different but this is the main idea on how to record for a directional instrument.
Does this help?
Where do you get the 30.5" from in the mean D&R column? Also, I remember doing some additional math if the direct angle to the foresight was > 180°.
Field Dog the 30.5 comes from the mean of your Direct on the FS and the Reverse on the FS.
28" + 33" = 61 and 61/2 = 30.5 or the difference between D FS and R FS; 33-28=5 5/2 = 2.5
and 2.5 + 28 = 30.5
You say you remember doing some extra math if the direct to the foresight was less than 180.
?? your direct to the foresight is less than 180. Its D = 94-53-28 You will have to give me some more information.
Thanks for clarifying the math! In the past I remember situations where the FSD angle was greater than 180°, and some additional math was needed to mean it and the FSR angle. I maybe mistaken, so I need to dig up some old field notes, if I have them, and verify that.
Field Dog; I think you might be talking about something like this.
If the FSD was 190-11-12 then your FSR is going to be (say) 10-11-06.
FSD 190-11-12 FSR 10-11-06 the recorder checks to see if the reverse and the direct are 180 deg. different (approx.).
He can do this several different ways. All of this will be done by quick math in his or her head. 190 + 180= 370
370-360 = 10 deg or you can look at the FSR of 10 deg and add 180 and get 190 and you see that the degree part is correct. Visual inspection one also see's that the minutes are correct and looking at the seconds; are close.
Now what is close; well by running your instrument (turning angles) over days and weeks you get a feel for how much the spread is between the D&R. Many things affect the spread, plate bubble going out, weather conditions and so on.
In turning angles I would suggest that you start with (around) 30 seconds on your inst.. This is because if your Reverse on your starting point comes up less that 180 deg. you have (or could have) a little problem in meaning.
This is because if your Reverse on your starting point comes up less that 180 deg. you have (or could have) a little problem in meaning.
Are you trying to say that a poor angle set will be more apparent?
One night operating the T-3 I started at 0°00'40±". After reading 6 sight directions direct, flopping over and reading them back to the origin I came up with 179°59'58.6", and I was called names I couldn't post here!
You were off by 1.4" and they threw a fit? Is the T-3 accurate to 0.1"? Is the T-3 operated like the T-2, where you don't set zero, but something close to it? I've used a T-2 before, but not in the way or for the work that it was intended for. It was simply available for use. It was a beautiful instrument, even though the mirror had a crack in it. I've seen a T-3 before, and I was told it was used to layout an Amtrak rail on a spiral curve at night using Polaris observations. I have never been fortunate enough to have been involved in such a thing, but would love to experience it at least once. Is this kind of surveying still done, or has it been trumped by GPS surveying?