Hello.?ÿ It's been a few decades since I've used a full-station instrument to obtain spot elevations.?ÿ I didn't have any trouble using the instrument, but I could use some help reducing the data.?ÿ I recorded SD, HD, VD and prism height in the field.?ÿ Shot a known benchmark (backshot) to begin, then shot spot elevations (foreshot).?ÿ Having difficulty doing the computations to find height of the instrument and spot elevations.?ÿ Any help would be appreciated.
Draw a sketch showing your distances (slope, horizontal, vertical) and the solution should jump out at you.
Don't forget that a negative vertical distance to the backsite is to be ADDED to the rod height and the bench mark elevation to get the elevation of the instrument.
From there you can add algebraically the vertical difference and subtract the rod height from that to get the foresight elevation.
Rinse. Repeat.
OK. That brings back memories. I actually calculated the HI correctly. Wouldn't have figured out the foresight elevations without your input. The final numbers look mostly correct. Will have to go through them again. Thanks.
Instrument height is not something you should calculate from field data, it is something you record as field data. However since you shot a backsight benchmark for elevation you can calculate your instrument elevation, but you should not now enter an instrument height as known field data. You record it with a full explanation of how that information was derived. From time to time, I have had to recheck my raw data as I hurried and did not record it in the field book. Thankfully my long used data collector SMI ?ÿwould never let me take a shot once I had set on a new point without my first inputting a new HI.
A few weeks ago my Carlson data collector would not link up with the instrument via the cable or by Bluetooth. I can only think some Bluetooth source was interrupting the whole process. I went through the settings on both multiple times to no avail. I hand recorded every part of every shot. I actually had more data than I ever had way back when, because the total station easily spit out Horizontal Angle, Zenith Angle, Slope Distance, Horizontal Distance and +/- Elevations. On traverse shots I calculated distance from ZA & SD, but on the topo shots I just took the HD and +/- with my HI - HR. Did a few both ways and yes, the instrument was always correct. It was fun and I may do a small job with my Topcon 20" Theodlite and a Philadelphia Rod. Week before last I took out an HP 48 with a port problem but with SMI cards and manually entered data, 130 points, recording only my traverse and tie shots in the field book.
Friday I got a call from a surveyor I worked with on many, many jobs to help him get a dwg back on his original SPC system after it passed thru a few too many engineer's hands with a loss of all points. Actually that part of it took less time than purging the gazillion layers and blocks added to it. He is in the process of packing up his office and returning to his home and offered anything I wanted. Having much more than I needed already, I grabbed an oversize target, and a few odd tape colors that I typically do not use. He then pulled a brand new 20 year old HP 48 in original package off a bookshelf, the battery dates inside the package were 2003 and they had leaked. Then he grabbed a black TDS box for it that I used back in 1994. I am in seventh heaven. Added new batteries, switched my SMI cards and I am back in my comfort zone.
Just pushing buttons is very boring.
Paul in PA
Using a total station does not require knowing instrument height unless you are using software that dictates doing so.?ÿ Everything is relative so long as you don't change prism height.?ÿ If you do change prism height all you have to do is add or subtract that differential from your standard numbers, once you measure that change in prism height.?ÿ ?ÿ For example:?ÿ Prism all the way down might be your standard and prism all the way up might be your alternate height.?ÿ Easy as pie.
Using a total station does not require knowing instrument height unless you are using software that dictates doing so.
If you start with a bench mark back sight (real or project 100) it's just like using a level except that the instrument takes care of reading the "rod".?ÿ
The alternative is to set up over the bench mark and measure up from it to the instrument height, but that is likely to be less accurate.
To know the difference in HI & HR height, one needs to first know the HI & HR height. Simple as that.
For general surveying I know of no directions that indicates one measures the difference between the instrument and target only. One can use software or a field book and do all manner of surveying without knowing any elevations. But having done that one cannot then go back and create accurate elevations from that data.
For myself I carry elevations on all my traverse work, whether or not I do it on some or all sideshots. At the end I typically expect my elevation closure to as tight or tighter than my horizontal closure. In doing that all my backsight shots also include elevations. Unless I need higher all my rods come out of the vehicle and set up at a minimum 5.00'. On level ground with a point at grade, my HI is 5.2'-5.3' and if set a pism on my tripod it is at 0.31' less than my HI, because that is how all my equipment is set up. A good number of my traverse points are set 0.2'-0.3' below grade on earth and my HI when I set a mag in a curb is always lower. As an engineer and surveyor the majority of my work requires elevations at some point and my typical job is 100% elevations except for 1/2 my laser shots. I leave a lot of crow's feet 5' above grade on a lot of siding.
Paul in PA, PE, PLS
It is all relative.?ÿ Shoot the bench mark first.?ÿ Then shoot some random point.?ÿ Say the bench mark has a vertical distance of 1.00 foot lower than the total station while the random point has a vertical distance 1.00 foot higher than the total station.?ÿ The random point is 2.00 feet higher than the bench mark.?ÿ It does not matter what the HI is.?ÿ The key is to keep the distance from the center of prism to bottom of rod tip the same.?ÿ The elevation of a point directly below your instrument set up is irrelevant in the general case.
I'm not a professional surveyor. I took a course in college about 1983 and used it from time to time at work when I needed to determine the relative elevation of the groundwater monitoring wells. We refer to it as "top of casing" elevation. We use this elevation to measure down and determine groundwater elevation, and then direction and gradient. Generally, I use a transit and stadia rod (or auto level and rod) to determine elevations based on an arbitrary (assumed) elevation at one of the monitoring wells, but I have used a full station in the past because that was what was available. In this particular case I rented a full station because two of the MWs were down a ravine about 30 ft lower in elevation than the other MWs. Never have I measured or recorded the HI in the field. I've always determined HI after taking the first backsight to the reference datum.
Elevation of Instrument Point + Measure up of Instrument + ((COS (Zenith Angle)) * Slope Distance) - Target Height = Elevation of Foresight Point
GROUNDWATER MONITORING WELL CONSTRUCTION SPECIFICATIONS
Standard Operating Guidance Updated September 2011
4.0 SURVEYING
A licensed surveyor will survey well elevation. Survey point(s) will include:
- concrete pad (marked with a spike);
- outer protective steel casing, when open (engraved mark);
- inner PVC well pipe (engraved mark);
- ground surface (not marked);
- well location to within + 0.5 foot in horizontal plane;
- ground surface elevation to within + 0.01 foot;
- surveyor’s pin elevation on concrete apron within + 0.01 foot;
- top of monitoring well casing elevation to within + 0.01 foot; and,
- top of protective steel casing elevation to within + 0.01 foot.
Using a total station for elevation requires a ground control point with elevation or a benchmark backsight. To get the required 0.01' precision on total station shots requires a D&R observation, especially at any substantial zenith angle. I shoot a lot of monitoring wells, and always shoot in order MW casing/mark, MW Rim amd grade shot North of MW concrete. Grade shots are to pavement/concrete/gravel/grass/etc. In addition I hand measure top of casing to rim and record on my field notes. I verify that my elev. to elev. shots are within 0.01' of my hand measured difference before moving to next well.
Paul in PA
Yeah, I always shoot to a "benchmark" to begin. Most of the time I assign an elevation value to that benchmark because we don't need to reference actual elevations, just relative. In this case, I was lucky to have a drawing with a known elevation, so I used it as my bench.
I don't know what D&R means, or zenith angle...and all due respect, don't really want to know.
Is there a relatively inexpensive instrument that will determine elevations to within 0.01' using GPS or other technology besides standard survey equipment. Not even sure if a $20K GPS system will do that.
Yeah, I always shoot to a "benchmark" to begin.?ÿ Most of the time I assign an elevation value to that benchmark because we don't need to reference actual elevations, just relative.?ÿ In this case, I was lucky to have a drawing with a known elevation, so I used it as my bench. ?ÿ
I don't know what D&R means, or zenith angle...and all due respect, don't really want to know.
Is there a relatively inexpensive instrument that will determine elevations to within 0.01' using GPS or other technology besides standard survey equipment.?ÿ Not even sure if a $20K GPS system will do that. ?ÿ
"Is there a relatively inexpensive instrument that will determine elevations to within 0.01' using GPS or other technology besides standard survey equipment"
No!
And even the "standard survey equipment" requires a well trained and knowledgeable operator to return 0.01' accuracies over an area much bigger than a tennis court (assuming that it's a reasonably FLAT one at that).
Loyal?ÿ
?ÿ
Is there a relatively inexpensive instrument that will determine elevations to within 0.01' using GPS or other technology besides standard survey equipment.?ÿ Not even sure if a $20K GPS system will do that. ?ÿ
A GPS System will not do that at any price. To do it consistently is a job for differential levelling, and even that will need to be handled with care.?ÿ Understand that while all instruments will return results out to as many decimal points as you tell it to, there is a limitation on how many of those are significant.
