Last year I did a topo in a parking lot where IÛªve been surveying for 30 years. Everything I did was routine, but something went wrong and parts of the topo are incorrect. I canÛªt see a flaw in anything I did, but something obviously happened.
I always follow the same plan. Set the base up, shoot key points including the bench mark with a bipod for stability, remove the bipod, and start collecting. Absolutely standard and IÛªve done it thousands of times.
I uploaded the original untouched archived topo file from last year and tried to duplicate what I did. Half the site is torn up, but outside the fence the pavement is untouched. Same as last year, I calibrated on my key boundary monument from 30 years ago, shot a second boundary monument, the bench mark, the PK control points that are still there, and took pavement and concrete elevations. (The PK control points were needed for the later reflectorless shots for the areas under the trees and by the building.)
Here is the mystery: Five of the control points I shot last year using a bipod for stability are still there and match todayÛªs shots within 0.03 feet vertically. All of the topo shots I took last year without a bipod are low by 0.45 feet average. All of the reflectorless shots are correct. You could conclude I had accidentally changed the rover rod when I took off the bipod, but I am obsessive about checking the rod height and surely I would have noticed after several hours, and in any case I would never change the rod height a piddly half foot. I have a standard rod height of 5.60 feet for almost everything and make sure the markings face me.
The bottom line is I have no idea what gremlins struck, or how.
Errors that are divisible by nine can be attributed to a transposition.
But don't feel bad. The other day I downloaded a bunch of corner records and took off for the job. Trouble was I was one township too far north (8 north instead of 7 north). And the hilarious thing was I started to get things to fit...until I noticed the correct street name on the corner reference was NOT where I was at.
We all blow it now and again. But as you've discovered: when you can't pin point what happened it is very unsettling to folks with brains that work like ours.
Clean it up and move on...and quit looking out the airliner window...(reference to Bill Shatner's Twilight Zone episode) 😉
If Bruce had a 5.60 roof height he would have had to enter a rod height of 6.05 to get points 0.45 low. That is a double transposition, not unusual because I have seen it occasionally. However a transposition should be self evident in the raw data file.
If Bruce entered 5.60, but the rod collapsed to 5.15 which is not an unusual minimum for a standard rod, pointa would be 0.45 too high.
I like the 5.00 minimum rod height and use it for all three rods. I have a three section rod, that collapsed with a 3" tilting prism without the space is 5.00. hen I have a two piece rod that has a collapsed reading below 5.00 feet that I keep promising myself I will take apart and shim so that it bottoms out at a 5.00 reading. I have to adjust a 3" tiling prism on the 5/8" threads to get the 5.00 height to prism. My third rod is a three piece shorty, that bottoms out at 2.70 with a mini prism. it almost always has a 2' extension between the rod bottom and the point, but there are times when I remove the extension to get a low shot. My wife is used to adding the 2' to the rod reading when I check my rod heights. I like to carry that shorty with the extension separate in my lath bag when traversing through the woods.
Paul in PA
[USER=20]@paden cash[/USER]
I was really scratching my head recently attempting to locate a quarter section corner from the corner record. The fortunate thing was I had found two corner records for that corner. I pulled out the other corner record and things came together very well. I looked over the first corner record again and then it hit me. The references were fine, but they were for section corner one-half mile further east. One of the reference ties was to the southeast corner of a bridge deck. Helps to have a bridge deck near your corner when the reference tie says there is one.
Slant vs ARP maybe?
Shawn Billings, post: 418027, member: 6521 wrote: Slant vs ARP maybe?
That's the first thing I thought of too. That difference for our receiver is about half a foot.
Bruce Small, post: 418005, member: 1201 wrote: Last year I did a topo in a parking lot where IÛªve been surveying for 30 years. Everything I did was routine, but something went wrong and parts of the topo are incorrect. I canÛªt see a flaw in anything I did, but something obviously happened.
I always follow the same plan. Set the base up, shoot key points including the bench mark with a bipod for stability, remove the bipod, and start collecting. Absolutely standard and IÛªve done it thousands of times.
I uploaded the original untouched archived topo file from last year and tried to duplicate what I did. Half the site is torn up, but outside the fence the pavement is untouched. Same as last year, I calibrated on my key boundary monument from 30 years ago, shot a second boundary monument, the bench mark, the PK control points that are still there, and took pavement and concrete elevations. (The PK control points were needed for the later reflectorless shots for the areas under the trees and by the building.)
Here is the mystery: Five of the control points I shot last year using a bipod for stability are still there and match todayÛªs shots within 0.03 feet vertically. All of the topo shots I took last year without a bipod are low by 0.45 feet average. All of the reflectorless shots are correct. You could conclude I had accidentally changed the rover rod when I took off the bipod, but I am obsessive about checking the rod height and surely I would have noticed after several hours, and in any case I would never change the rod height a piddly half foot. I have a standard rod height of 5.60 feet for almost everything and make sure the markings face me.
The bottom line is I have no idea what gremlins struck, or how.
Do the rod heights in your raw data file check against your field notes?
Kevin Samuel, post: 418041, member: 96 wrote: Do the rod heights in your raw data file check against your field notes?
Bruce doesn't keep field notes. But an examination of the measure ups in the raw data file is in order.
Mark Mayer, post: 418043, member: 424 wrote: Bruce doesn't keep field notes. But an examination of the measure ups in the raw data file is in order.
The blunder could have occurred in the past, or it may have occurred on the recent field excursion. Ideally the measure ups in both raw data files will be scrutinized.
Did you check to see if you used vertical or slant height when you entered HR or HI....
Mark Mayer, post: 418043, member: 424 wrote: Bruce doesn't keep field notes. But an examination of the measure ups in the raw data file is in order.
Can't be a measure up because the error took place after the base was up and running and I had taken all of the control shots. The control points then and now match within hundredths. The error began when I removed the bipod and started regular topo shots. Same rover rod, same rod height. Nothing changed, except obviously something went wrong.
All of the reflectorless shots, which used the rover control points taken with the bipod, are correct.
Once I had a similar problem. Set up base checked in goor and started staking points. During lunch the data collector went to sleep and I turned it back on and restarted the robot selecting "use last." Checked in bad by about 0.6' into a control. Double checked measure ups and rod heights and they where on. Well eventually figured out some reason the DC had reset itself to use for the measure up the "true height" mark instead of the "bottom notch" mark on the S6.
Are your topo shots taken from the same measurement mode as your control?
If not, isn't there an opportunity to input an incorrect measure up, or simply forget to enter the measure up (in which case most software I have used defaults to measure up last used and entered within that particular field)?
Does the module you use for topo allow for an offset (perhaps on a menu not readily visible)?
Do you ever change your measure up when collecting topo to force the elevation of shot to be lower/higher? (Think the case of a clogged culvert. I don't condone it, but I know it does happen, some guys really hate using a shovel).
Kevin Samuel, post: 418065, member: 96 wrote: Are your topo shots taken from the same measurement mode as your control?
If not, isn't there an opportunity to input an incorrect measure up, or simply forget to enter the measure up (in which case most software I have used defaults to measure up last used and entered within that particular field)?
Does the module you use for topo allow for an offset (perhaps on a menu not readily visible)?
Do you ever change your measure up when collecting topo to force the elevation of shot to be lower/higher? (Think the case of a clogged culvert. I don't condone it, but I know it does happen, some guys really hate using a shovel).
Good thoughts, but none of them apply. I'm careful and conservative when data collecting, never anything fancy. Standard procedures, plain and simple, day after day, just so this won't happen. I'm still mystified. The same procedures day after day should produce the same results day after day. I'm replotting everything to compare then and now, and maybe that might provide a clue.
I appreciate your input. Maybe tonight when I'm having a shower and my mind is relaxed I'll have an "aha" moment of clarity.
That is how the mystery is solved many times. Get distracted by something else and then.............whammmm.............there's the answer.
I thought I was the only one this happens to, inexplicable, unintended "results". Now I see there are two of us. The answer is that no mistake was made. Sometimes we just have to deal with snafus that happen for the sole reason of testing our frustration. It's the only possible answer.
Perhaps you lost initialization and when it came back you got a bad initialization?
David Powell, post: 418079, member: 6431 wrote: Perhaps you lost initialization and when it came back you got a bad initialization?
The Leica reinitializes automatically every ten seconds, so one bad shot maybe, but not all of them.
Do you check back into your control at the end of the day?
voidintheabyss, post: 418083, member: 11972 wrote: Do you check back into your control at the end of the day?
To my regret, I did not, and that would have caught it.