land surveying

[chickx]

when carrying out land surveying to ascertain levels what does it mean if your to take a "TBM" from a specific point

[heathfield]

TBM is a ''Temporary Bench Mark''

[chickx]

some surveyors have been "getting levels"next to my house, i've just been looking at their little yellow book they left and became interested in it all, unfortunately i dont think they are coming back any time soon and i cant find any info on the net or local library so could any one answer the following questions

what is the TBM for and how do you do it,
what is backsight and foresight
and what is collimation or HPC

[KARL]

Levelling by optical instrument where this terminology is used is based on horizontal sightlines. You do your work by looking at a staff (a vertical, rigid measuring tape, if you will) positioned ontop a point of a known elevation (height above mean sea level). That reading is the backsight, the staff then being moved to the TBM and another reading taken, the foresight. Thus by applying the difference in the two readings you can deduce the elevation of the TBM. You can now move the instrument and take a backsight onto it (giving you the elevation of your sightline) and examine other points (foresight again), TBMs or not - there can by lots of intermediary readings before you move onto the next TBM (or pack up, job done)..

[chickx]

so i set the tripod up, take some levels from that point, determine the elevation of another point which i will use as my TBM and then take other levels from that point, would you normally do the same points from different TBM's to make sure your levels are correct?

[heathfield]

TBM: Ordnance Survey would establish Bench Marks to mark a known elevation at a known point above sea level. A building firm is not concerned with sea level, but simply wants to map a small land area. So they can set a TBM from which all other measurements of the site can be made.

Backsight and foresight (Paragraph 4) is looking behind you to the last measured point, and ahead to the next point of interest respectively.

Collimation is making sure that the vertical axis of the theodolite is optically aligned to a true 90� to the horizontal.

HPC: Height of Plane of Collimation. This is the height of the instrument with which observations are being made. Take a backsight to the benchmark and staff as in the diagram shown in the backsight/foresight link. This will establish the height of the HPC. By taking a foresight to another point, and calculating the difference between the HPC height and the new measured height will give the height of the new point relative to the benchmark.

Took a while to write, and I see Karl's got in ahead of me!

[KARL]

To establish the degree of error (if any) it is common to 'close' by arranging to eventually end back where you started. If the calculated level you arrive at is not the same as you started with, then you have the difference as your error. If minimal it may be possible to ignore it, but in geodetic surveying (serious stuff) you often have very large loops to reconcile and it is common to distribute the error over all the readings unless it is significant enough to warrant a re-survey.