Archaeology and the Total Station

The main purpose of using a total station for archaeological fieldwork is to impose a grid (X-northing, Y-easting, Z-elevation) on the site to locate every artifact and feature in 3-D space and to make topographic maps of the site. Every site has a site datum that spatial measurements are referenced to. Topographic maps record the position of natural and human-made features on the earth’s surface. Some features that are mapped at an archaeological site include elevation, streams, roads, excavation units, artifacts, features, the datum, site boundaries, and any other important features. Many topographic maps use contour lines to represent terrain. A contour line is a line that passes through points with the same elevation. Other features are usually represented by symbols. Data collected by the total station can be downloaded into a computer and mapped using mapping software.

A total station (a.k.a. EDM, transit, theodolite, the instrument) is the ultimate in survey instruments. A total station combines a digital theodolite and an EDM that work together with a microprocessor to rapidly and accurately perform tasks. With this combination, the total station can measure horizontal and vertical angles, slope, and horizontal and vertical distances. A total station also has a built-in calculator that performs trigonometric calculations, as well as an electronic field notebook used for storing data. The total station can interface with a computer for data transfer.

EDM (Electronic Distance Measuring Device): An EDM is a device mounted on the top of the total station that sends out a light wavelength. The EDM measures the time it takes for the light to travel from the EDM to a prism, bounce off the prism, then return to the EDM. This measurement is displayed as distance, but it is really a measurement of time. Very accurate distance measurements are taken using an EDM.

Theodolite (Transit): A theodolite measures horizontal and vertical angles. Horizontal angles are measured along a level horizontal axis. Vertical angles are measured along a level vertical axis.

Tripod: The base that the total station instrument is mounted on.

Plumb Bob: Used to center the tripod over a point (an optical plummet on the instrument can also be used).

Total Station Instrument: The electronic unit that combines the EDM, theodolite, and field notebook.

Prism and Rod: A prism is mounted on a graduated rod. The EDM reflects light off the prism. The rod has a circle level to keep the rod plumb.

General Field School Total Station Procedures

Each day one student is assigned to set up and dismantle the total station (I will help). Total station setup assignments rotate daily. At the beginning of each day this person sets up the tripod, total station, and prism; levels the instrument; turns on the instrument; creates a Job; conducts the instrument station setup; shoots in the laser level; and fills in the information on the top of the EDM form for each Job. At the end of the day this person turns the total station off and saves the settings and dismantles the instrument, tripod, and prism.

Each student is responsible for running the instrument (after they feel comfortable with performing this task without help) when recording their own shots (taking the measurements of their point while someone else holds the prism). Your crew leader (or me) will help you take measurements and record data. The data that a student collects using the total station is used to map units and profiles. The total station has one Job every day. A Job organizes all recorded data into a database. Each day a new Job is created (labeled with the day’s date) that is used when recording shots. When recording a shot, the student must record their point into the day’s Job, enter in the point number, and enter their initials after the code (e.g., BF/KLT). Each point information is recorded manually by the student into the EDM Log and into the total station. At the end of each day I will download all the information from the total station to the computer.

1. Roughly set the tripod over the point by holding two legs and set the third past the point. Move the two legs that you are holding until the top of the tripod is approximately over the point. Make sure that the top of the tripod is roughly level. Press the legs of the tripod firmly into the ground. Screw the instrument onto the tripod. Make sure that the leveling screws are even. If they are not even, screw each one of the screws so that it is in the middle of its leg.

2. The plumb bob can be used to position the instrument over the point. The optical plummet is quicker and, in my opinion, easier to use than a plumb bob. Look through the optical plummet and determine its location relative to the point (you sometimes have to place your foot next to the point to determine its location). Adjust the location of the tripod (if necessary) to find the point in the optical plummet by lifting two legs and pivoting the instrument on the third leg while looking through the optical plummet. Center the optical plummet exactly on the point.

3. Level the tripod base. Carefully adjust the tripod legs (one at a time) up or down to center the circular level on the instrument. Check that the optical plummet is still on the point.

4. Level the instrument plate bubble using the plate level.

a. Align the level so that its axis is parallel to the line between two of the leveling screws.

b. Observe the location of the bubble. Move both leveling screws with your thumbs equal amounts in opposite directions. Center the level exactly between the two large lines.

*Left Thumb Rule: Both thumbs move in or both thumbs move out. The level follows the direction of the left thumb.

c. Rotate the instrument 90 degrees so that the bubble is aligned over the third screw. Use that screw only to center the bubble exactly.

d. Turn the instrument to its original position (as in a.). Level again if necessary. Continue this process until the bubble stays centered as the instrument is rotated.

5. Check the optical plummet. If it is slightly off of the point, loosen the instrument’s attachment screw on the tripod. Carefully slide the instrument exactly over the point while looking through the optical plummet. Do not rotate the instrument. Tighten the clamp. Then re-level the plate level.

1. Attach the prism onto the top of the rod.

2. Place the tip of the rod’s base on the center of the point that will be measured.

3. Usually the height of the rod should stay at 1.5 m. This is the distance from the tip of the rod that is on the point to the center of the prism. The lower the rod is, the more accurate the measurement. If the person running the instrument cannot see the prism, then you should raise the rod. Tell the person running the instrument the height of the rod or target (HT). The person running the instrument will use the HOT key to change the height of the target.

4. Position the prism so that it is facing the instrument. Hold the rod so that the circle bubble is centered.

5. Keep the prism in position until the person running the instrument says "Okay" or "Good."

Sighting a Point

Unlock the vertical and upper clamps. Sight in the reflector so that the crosshairs on the telescope are in the center of the prism. To do this, use the optical sight (Finder). Align the optical sight arrow with the prism rod or prism. Lock the upper clamp. Look through the telescope and find the prism. Lock the vertical clamp. Use the fine-tuning knobs on the ends of the clamps to center the crosshairs on the center of the prism.

The following is a daily step-by-step procedure of how to use the total station during excavation after the instrument is leveled.

1. Turn on the Total Station. Press the [power] button. The person assigned to instrument setup will perform this step.

2. Create the Job. Press the [menu] key. Press [1] Job. Press the [menu] key. Press [1] create. Input Job name (for example: 62802). Press the [enter] key.

3. Set the Horizontal Angle to 0. Make sure that the EDM laser is on top of the instrument. Sight in a point along the N-S baseline looking north. Sight the crosshairs on the bottom of the rod. Lock the horizontal angle (use the upper clamp). Press the [ANG] key. Press [1] key for HA 0-set. The horizontal angle is now 0.

4. Instrument Station Setup From a Known Point. A point is assigned at the site that the total station will be set on every day during excavation. The coordinates of this point will be known in relation to the site datum. The site datum’s coordinates are X:1000 m, Y:1000 m, Z:1000 m. The instrument station must be set up before any data points (shots) can be recorded.

Press the [STN] key to display the menu screen.

Press the [1] key for "known" coordinates. A screen is displayed for input of the station point information (ST: ). The station point is the point that the instrument is over. Input "1" for the station name and the coordinates of the point. If the point number/name has already been recorded its coordinates are displayed. If the input point is new, the screen waits for the input of coordinates of the point that the instrument station is set on in relation to the site datum. We will know these coordinates. Input the height of the instrument (HI: ). Measure (in m) the height of the instrument from the ground to the center of the eyepiece. Input the code (CD: ) "ST" for the station point. Press the [ENT] key. Press the [ENT] key. The screen changes to the menu display for methods of setting the backsight point.

The screen menu still displays methods of setting the backsight point. Press the [1] key to select "coordinates." Input the backsight point name (PT: ). The backsight point name for the point that we use for the excavation's backsight point will always be the next point after the station point (or 2). Input the height of the target (HT). The height of the target is usually 1.5 m, but this might change. Input the code (CD: ). The code is always "BS" for backsight. Enter the coordinates of the backsight point. Press the [ENT] key. Set the prism on the backsight point and sight in the prism. Press the [ENT] key.

The station is recorded and the HA (horizontal azimuth) is calculated between the two points from the input coordinates. The BMS (Basic Measurement Screen) is displayed but no coordinates are displayed. Now all of the points measured will be referenced to the site datum.

5. Perform the Backsight Check. To make sure that the total station is on the grid system, take a measurement of a known point (see #7 for measurements). If the measurement is correct then record the shot as the next consecutive point number after the backsight and the code BAC. If the measurement is wrong then the total station should be re-set up.

6. Shoot the Laser Level. On the first day the laser level will be tied to the site datum (we will calculate the distance below the datum). The X, Y, Z will be recorded relative to the datum. The student assigned to the total station setup will measure and record the laser level before any other measurements (or shots) are taken and before the laser level is turned on. The Z of the laser level is set to 1001.13. This is the height of the laser level relative to the height of the site datum. The rod person places the center of the prism exactly on the location that the laser is emitted. The height of the target is now 0, so the person running the instrument will use the HOT key to change the height of the target. The laser level must be set to 1001.13, so the rod person must move the laser level up or down then wait for the person running the instrument to measure and read the Z coordinates. The rod person adjusts the laser level, and the person running the instrument will measure the elevation (Z) until it is 1001.13. The coordinates of the laser level are then recorded. The person assigned to the total station setup performs this task. The code for the laser level is LL. The total station is now ready to take measurements.

Take the Measurement: Make sure that the X, Y, Z are displayed on the BMS menu. If they are not displayed then press the [DSP] key until they are displayed. Have the rod person position the prism on the point. Sight in the center of the prism on the crosshairs. If the height of the target is not 1.5 m, press the [HOT] key. Select HT and input the height of the target that the rod person reads to you from the prism rod. Press the [MSR] key.

Record the Measurement: Write the X, Y, Z coordinates on the EDM form. Press the [REC] key to record the measurement into the specified Job’s database. Raw and coordinate (XYZ) data are recorded. The point name input screen is displayed. Enter the point information for the point name (PT: ), such as "8." Enter the height of the target (HT: ) (a.k.a. height of the rod that the prism is on, usually 1.5 m). Enter the code (CD: ). Use the code list to identify what the point item is. Enter the code for that item followed by a / (slash) followed by the initials of the student who is taking the shot (to know whose point the shot is) (e.g., BF/KLT). Copy this information onto the EDM form. Write any comments about the shot on the EDM form that might help you when mapping. Be sure to press ENTER.

8. Putting the Instrument to Sleep or Turning the Instrument Off

Sleep: After taking a measurement, and if no one is waiting to use the instrument, put the instrument to sleep. Press the [PWR] key, then press the [4] key to put the station to sleep. Make sure that the upper and vertical clamps are unlocked and the lens cap is on the eyepiece.

Off: To turn the instrument off, press the [PWR] key, then press the [ENT] key. This will save all of the instrument settings.

Miscellaneous Useful Screens, Commands, and Keys

BMS Screen
The BMS (Basic Measurement Screen) is where all the measurement data is displayed. This screen is usually the default screen when the total station is not performing any functions. Press the [DSP] to change between the types of data displayed.

[ESC] Key
This is probably the most useful key. Press this key if you want to abort an operation, get out of a screen, etc.

Arrow Keys
The arrow keys can be used to scroll across and up and down the screen.

Deleting Text
To delete letters or numbers on a screen put the cursor on the number or letter, then either type over the text or press the [ESC] key. Or place the cursor to the right of the text and scroll back (left arrow key).

Typing Text
Letters are shown above the key and numbers are shown to the left of the key. Press the [mode] key to change the input mode between alphanumeric and numeric. In alphanumeric mode a box with an "A" is displayed on the right side of the screen. Letters above the keys can be input. In numeric mode a box with a "1" is displayed on the right side of the screen. The numbers to the left of the key can be input. Use these keys to input code, point, and coordinate data.