SURVEYING FROM AN UNKNOWN POINT
WITH
LEICA TOTAL STATION
The total station can be used to survey in locations where you do not know the value of the point upon witch it is setup upon, this is known as Free Station Surveying. This survey technique is often used when you have two points with known established coordinate values but are not in an ideal place to perform your survey from, or at times where you wish to progress further into an unknown survey region.
A good example for using this technique would be if you needed to measure points under a dense forest canopy where your GPS could not provide high precision point values. In a case like this you would use the GPS unit to establish known values around the perimeter of the tree stands and then use them as back-sights so that you can setup your total station in under the forest canopy.
You would then be able to establish the value of this unknown point and then continue collecting other unknown points as well. Accurate results from this style of surveying rely on careful planning and the use of good geometry when setting up your known point.
Set up is similar in many ways to the set up technique explained earlier when surveying with the total station setup over a known station. Therefore we will assume that you can follow the steps outlined above in earlier sections for instructions on setting up the total station, leveling the unit and measuring the HI.
– Create the measure job, data job and select codelist as outlined previously (all the steps are the same).
– Once the above steps have been taken return back to the main menu and press the PROG key from the bottom row of the key pad.
– Next select Free Station from the program Selection menu by pressing 1 on the key pad.
The station ID is a uniquely identified for the point that you are occupying. It is good practice to use the point id values of the two known points that you will be measuring as this identifier. Something like pt1 or pt2 can also be used.
– Enter the instrument height (HI) of the total station. Press CONT by using the F1 key.
Now you will be on the Target Point screen.
– Enter the ID of the first known point you wish to use. Enter height of reflector pole with the prism above the unknown point. Aim the total station towards the reflector.
Then shoot the laser at the reflector with SEARCH by hitting the F1 key.
On the next screen press F2 to use the DIST function. Keep pressing the using the DIST function until the number you see is resolved to within 5mm of the last two DIST measurements. After you are satisfied with the value that was resolved from the measurement process then Press F4 CONT.
Repeat these steps for the second known point.
Once these two points have been shot use the F6 button to access the CALC function. This will do the math for the total station to figure out where it is set up upon. The total station will know spatially where it is located so that it can measure and calculate more unknown points within the survey area.
Note: You may result with an error screen telling you the error calculation is too large. If this occurs you must try to resolve this by shooting to the two known points again to obtain better, more accurate data. What has occurred is that the place where you told the Total station that the two points are not matching up wit the math it does to confirm that and establish it own position.
If everything is fine you will arrive at a screen that will give you and option on the bottom left screen above the F1 key SET. Press F1 to establish the Total Stations position. This brings you back to the main menu.
You can now use the MEAS function to begin your survey as per the instruction in setting up over a known point explained above.