Doing Backsights and Foresights ©2010 Dr. B. C. Paul Note – The techniques shown in these slides are considered common knowledge to surveyors. Figures in the slides may be the authors own work or extracted from Instrument Users Manuals, Surveying by Bouchard, Mine Surveying, or various internet image sources.

Motions on a Transit Transit rotates on spindles Interesting thing to note is that there are Two of them Transit is freed to rotate by Loosening a knob We speak of the “Lower” and “Upper” Motion on the transit Knobs are either lower or upper Lower motion frees the transit to rotate but Does not change the angle reading plates Upper Motion allows transit to turn over The metal plates where vernier readings taken

Why Two Motions on a Transit? Common action in surveying is to sight down a line of known direction and then turn to a line of unknown direction The instrument will be used to measure the angle and thus provide the direction of the new line If you always turn your angles to the right just add the angle to your old direction to know the new direction What are the chances the vernier will read 0 when you are looking down the line of known direction? Do you want to do degrees minutes and seconds subtraction in the field?

Idea Use the Upper Motion and Set the Vernier to zero Lock the upper motion Now use the lower Motion (which does not change the vernier) to sight down the line of known direction Now it does read zero on the known direction line. Now use the upper motion to turn the angle right (and change the vernier to measure the angle).

What if there is no Lower Motion (like on our levels) Can Turn the Scale Circle and Set it on Zero for the backsight.

Measuring Angles with a Theodolite Theodolites Usually do not have a Lower Motion They are non-repeating instruments They usually have enough accuracy not to need the repeat trick Problem of how to get a Back-Sight Zero They have a knob or Wheel Used to Set 0

Typical Theodolite Rather than a lower and Upper motion can Zero scale with an Adjustment ring

Measuring an Angle with a Theodolite Site your back-sight or known direction Use the micrometer knob to set the fine scale to zero Use the zero knob or wheel to set the upper scale to 0 when you have coincidence Turn the angle right to your fore-sight

Review of Procedure If you have an upper and lower motion Set your reading to zero using the upper motion (and micrometer if applicable) Turn your instrument to your backsight using the lower motion Notice that you now have the angle set to 0 on your backsight.

More Review If you have an Upper Motion Only! Sight your backsight Note you are doing this first (you would do it after you set 0 if you had an upper and lower motion) Now use the micrometer and cheater knob to set 0 Or if it is a digital instrument push the reset button to 0 the angle.

Proficiency #4 Set Up and Level Instruments A Transit HW Theodolite T2 New T2 Old Total Station Digital Transit Level Pick a Backsight and zero your instrument on it Turn an Angle to a Foresight and Read it. Do it for each instrument