Learning to Survey with the Laser Total Station

Introductory Workshop

5 June 1995

Instructor: Dave Richardson

OUTLINE

** additional equipment needed: pencil, notepaper, measuring tape, compass (unless working with both established benchmarks and initial backsights).

I. Survey Objectives

Ia. X,Y,Z coordinates

Surveys using the Pentax Laser Total Station and SC-5 Datalogger are designed to give the surveyor both coordinates and elevations for the individual survey points. The coordinates can either be in feet or meters and can be given as Northings and Eastings (y,x typical of many private surveyors) or as x,y like for UTMs. Elevations are also in feet or meters. Generally x,y,z coordinates in meters are most useful for our surveys at the VCR. It is important to note that all coordinates and elevations are relative to the initial benchmark and backsight. Therefore, if real world coordinates or elevations relative to sea level are desired, the coordinates and elevations of at least the initial benchmark is REQUIRED (preferably coords and elevations for both it AND the initial backsight).

Ib. Coded information

Ic. Descriptive information

In addition to coordinates and elevations, you can store additional information about each survey point. This information can be whatever you wish, be it vegetation type to plot number to the name of a particular sampling well's owner. There are two "fields" this data can be entered into: one is a CODE field that accepts only numbers from 0 to 99, the other is a DESCRIPTION field that accepts character-number-symbol strings up to 16 characters long. Optionally the description can automatically include the text for what the code stands for.

It is important to think about what kind of information you plan on collecting before running off into the field. Information stored as a numerical CODE is often more useful than a text description if you plan on querying which points fit a certain class once the data is imported into a spread sheet, statistical package, or GIS. Two-digit CODE information also takes up less of the datalogger's disk space so that more points can be surveyed between downloading sessions. However, if you are collecting information that is unique for each point (like well owners' names) or that is a long list of possible combinations of information (like plant community species in order of dominance), the DESCRIPTION field should be used.

Often both fields will be used to collect different data about individual survey points.

Whether or not the program asks you to CODE or DESCRIPTION data, and whether it carries this information from one survey point to the next or else blanks the field for each new survey point, depends on what you have told the datalogger to ask for under "Setup" (IId. below).

Id. Accuracy discussion

The makers of the Pentax Total Station claim an accuracy of less than 2 cm. over a half kilometer distance in all x, y, and z coordinates. You will NEVER see this unless, like those making the claim, you eliminate all human involvement and permanently fix both the Total Station and the reflective prism. Sources of human error are: 1.) the exact measurement of the instrument height, that is, the height from the top of the benchmark or ground to the center of the Total Station's lens; 2.) the exact height of the prism on the stadia rod above the ground, accounting for any angle away from vertical at which the rod is held and how much if any the bottom of the stadia rod sinks into the ground. Both the total station and the stadia rods have leveling bubbles so that you can keep them vertical, and the stadia rods have different attachments for the bottom that include a sharp pointed tip and one with an extra wide flat round base (this one is especially useful when working on beach sand or mud). Generally error is 2 - 5 cm. in the x,y plane and just slightly more in the z. The exact range of error in any survey can be calculated ONLY if you perform multiple measurements on a collection of points as you go through the survey, such as measuring your backsight(s) as points at the beginning of the survey after zeroing and at the end as the last point as well, so doing this is heavily recommended.

II. Datalogger Initial Setup

IIa. Datalogger design

An illustration of the MC-5/SC-5 Datalogger (same thing, two different names) shows the keypad and the top panel where all the connector plugs are. Normally you will plug the cable from the DATA port on the Pentax unit into the COM1 port. Notice on the keypad that there are 2 shift keys. Hitting the SH1 key before another key will give you capital letters or the "upper" command printed ON the key, except for the function keys where SH1 first will give you the white function key above. Hitting the YELLOW SH2 key before another key will give you the symbol or command printed ABOVE the key in YELLOW. For example, in the case of the "A" key, hitting it alone will produce an "a", hitting SH1 then A will produce "A", and hitting SH2 then A will produce "\". The function keys preform the action shown above them on any particular screen. Notice that SH2-ON is required to turn the datalogger OFF. To adjust the screen contrast, you must use a fine screwdriver from the small tool case with the Pentax to turn the screw in the CONT hole in the top panel of the logger. To let air out from behind the keypad which may cause the plastic surface to bubble out (due to pressure change or expansion from being used in prolonged direct sunlight), just unscrew a pressure valve screw on the bottom of the datalogger a little until the excess air escapes, the screw back closed. MAKE SURE THE PRESSURE RELEASE SCREW IS CLOSED TIGHT, because otherwise moisture can get in and damage the circuit boards, and cause serious downtime for either you or the next group of people wanting to use the Total Station.

IIb. Checking available disk space on the SC-5

To check the available memory in the datalogger for storing your survey, turn the SC-5 ON and you should get the main menu of the Survey Assistant (SA) software. Then hit [SH2] [F2]. The following screen will be displayed:

Hit any key except the shift keys to return to the SA software. If there looks to be not enough room, you will have to delete existing surveys or "jobs" to free up space. PLEASE check on the 586 computer to see if the owners of those jobs downloaded the raw SA files before deleting their jobs (see creating/deleting jobs in IIc. and downloading in Va. below). If not, please download their raw SA files first before deleting them, and please notify the owner if known that their files were moved. Note that YOU are responsible for downloading both your raw SA files and the coordinate files, and that YOU should delete your survey job once you have sucessfully downloaded the files to disk to take back with you.

The following is an illustration and description of the SA Main Menu:

IIc. Creating a new survey or "Job"

To create a new Job, go to the Main Menu and move the cursor to "Job" (or just press "J") and you will enter the "Job" screen which will list the jobs currently in memory. To use an already created job move the cursor to that job and either hit "USE" [F1] or [enter]. To create a new job hit "CREATE" [F2]. You then give the new job a name and a name for the output file (the same). It is recommended that the name consist of either your initials and the start date OR the initials of the P.I. you are working under and the start date. For example, DLR956a would be the first survey that I (Dave Richardson) collect in June of 1995. This system avoids having lots of surveys named "HOG", "PHILLIPS", etc. where the owner can't figure out which one was his/her own at a later date. There is an 8-chacter limit on names, like most IBM-PC file names. The current job will then show up on the screen at the lower right of the Main Menu.

Under the "Job" submenu you can also delete jobs by selecting them an hitting "Delete" [F4].

IId. Check "Setup" to see how data will be collected

Although many of the subroutines have links to setup information within the subroutine, it is most convenient to check that all the setups are correct when first starting up from the Main Menu. Go into "SETUP" first. To check to make sure that the data will be collected correctly from the Pentax and from manual input, go into "Field Work". The various setups are described below:

Unless you know of some reason you require a different setup, we suggest that VCR datasets be collected using the following setups:
 

*note, unless you purposely hit the repetion key on the Pentax, you only use this stuff when conducting a mistake!
  The rest are environmental correction factor setups and wavelength coeffients that you don't need to mess with unless you borrow the equipment and take it someplace VERY different from the VCR (like high mountains or desert or artic climes).

The only other setup submenus regularly encountered are TRANSFER (thru COM1) and GRAPH which is just how to display points and will be dealt with when talking about graphing points in the field (IVe.)

IIe. "Edit" code list for new survey (optional)

Under the Main Menu choose the "View/Edit Job Data" menu, and choose "CODES" to get the following "Browse Codes" screen:

For example, if you are collecting data points and in addition to location and elevation you wish to delineate the points as being creek or marsh boundaries (so you can draw lines and divide those areas back in your GIS) you could create the following code list:
 

Then when collecting points along the creekbank, you can make them code 1, which will tell you that those points were along the creek, and so on. If you want the text you entered as that code's "name" entered into the DESCRIPTION field, type in the code number followed by "@" (This will be reiterated again later).

III. Instrument Setup in the Field

IIIa. Pentax design

The preceeding pages illustrated the parts of the Pentax Total Station.

IIIb. Setting up and leveling the Tripod and Total Station over the initial benchmark

The first step to your field survey is to set up the Pentax over your initial benchmark. If at all possible this should be an established benchmark, such as those installed either by NOAA or the USGS or by the VCR-LTER's GPS monuments. If no prior benchmark exists anywhere reasonably close to your survey area, you can either establish one yourself there with the GPS units BEFORE your survey, OR you can establish a benchmark and survey off of it and THEN go back and GPS it in. It is preferable to GPS your benchmarks first simply because otherwise you will have to add an x, y, and z correction to every point you survey after the fact, and in some other software package as well.

Once you have found the benchmark, set up the tripod so that the face plate is level and centered roughly over the center of the benchmark. You do not want the tripod legs so spread out that it becomes hard to walk around it without bumping the legs (which will make you relevel and rebacksight everything to continue), nor do you want it so high that the person(s) to operate the Pentax can not look through the eyepiece. It is often easiest when the ground is near level to adjust the legs so that with the legs together the faceplate is at eye level, and then spread the legs apart.

Set the Pentax on top of the tripod and screw it on loosely (so it can still move side to side a little). Make sure that to start with, the 3 leveling screws at the Pentax's base are adjusted to the middle and not fully at one extreme or the other (there is a yellow line that marks what the middle is and should lowered/raised to the point just before it dissappears). Adjust the 3 leveling screws until the circular bubble vial (level) is centered in the inner ring. Look through the Optical Plummet eyepiece on the side of the Pentax. If you are level and centered above the benchmark, you should see the benchmark's center below through the eyepiece. If not slide the Pentax around without moving the tripod and see if you can see the center of the benchmark. If you can not, tighten the Pentax clamping screw on the tripod, so the Pentax can not move, then move the tripod legs in the direction needed to put it above the benchmark. Repeat this as many times as needed to get the unit centered. Once it is, plug the SC-5 into the Pentax through the round port marked DATA, and place it in the attachment that'll allow the datalogger to sit on one of the tripod's legs. The weight of the datalogger is enough to unlevel all your fine adjustment work if you forget and put it on after everything's perfect.

Now use the fine leveling to perfectly level the instrument. This involves rotating the Pentax so that its keypad and the long leveling plate vial above it is between 2 leveling screws. Adjust those leveling screws (not the third behind) with small movements until the plate vial bubble is centered between the marks. Then rotate 120 degrees so the vial is between those 2 leveling screws, and repeat process, going around and around and around and around until the fine vial remains level between all leveling screws without you having to adjust anything. Confirm that you are still centered over the benchmark by looking again through the optical plummet. If all is well, you are ready to turn on the SC-5 and prepare to collect data.

IIIc. Measuring Pentax's height above benchmark

At present this is probably the most inexact part of your entire survey. Take your measuring tape (metric) and measure the distance between the top of the monument (or ground if this is a traverse) and the bottom of the tripod, sighting the bottom as level between the 2 gaps on either side of a tripod leg (SEE ILLUSTRATION ON NEXT PAGE). Do this 3 times, one for each leg-side, and write down the average. Now take the measuring tape and measure from the instrument center mark on the side of the Pentax to the bottom of the tripod face plate, the same bottom edge you used to sight the bottom when measuring up from the ground. Do this also 3 times and again take the average. Add the two averages together and round to the nearest 100th of a meter. This is the INSTRUMENT HEIGHT.

There does exist another piece that can be bought for the Pentax that will allow it to measure the distance above the ground/benchmark itself, and much more exactly, but at $5,000 we do not have it right now.

IIId. Entering Initial Benchmark's coordinates and elevation

Now before you turn on the Pentax, turn on the SC-5. (If it is a long distance to your backsight, send the prism-person with the prism hiking now.) Go to "Jobs" and select the job (survey) you want to start or continue. Back in the Main Menu select select "Field Work" to start collecting data. If you are continuing a survey, enter the point # of your occupy station and the correct backsight, and continue where you left off. If you are just starting a survey and have not yet collected any data, you will get your initial OCCUPY STATION screen:

The PT # should be 1. Enter the instrument height beside HI: . Remember that the height will be 1.xx meters, not 1xx centimeters. Change the date and time if not correct. Enter the correct CODE and DESCription for this point. If using named GPS monuments, please include the name in the DESCription. When done hit [F1] STORE. The logger will beep and tell you that the point does not exist and prompt you to enter your coordinates and elevation. Remember that Northing is the "Y" coordinate and that Easting is the "X" coordinate. If you are not working off an established benchmark, either enter 0, 0, 0 for the coordinates and elevation, or enter 1000, 1000, 100 is you can not stand to work with negative numbers. Again hit store or save when done.

IIIe. Entering Initial Backsight's coordinates and elevation OR initial azimuth

Now you will have a BACKSIGHT screen. This initially should be PT # 2. HT: is the ROD HEIGHT, the reading on the side of the stadia rod that says how high the center of the prism is off the ground (the numbers are correct for our type of standard prisms, so just read the number on the pole and don't worry about adding or subtracting anything). Initially the backsight height is unimportant because you generate a false point at the same elevation as the occupy station. OS=1 tells you that your occupy station is pt. # 1. Enter the code and description. The logger will again beep telling you that the point does not exist. Here you can either enter the COORDinates if known (including elevation), OR enter the BEARING. If you do not know the coordinates of your backsight, especially if there is not an established one, send the prism person out as close to TRUE north as possible and have them stand there. By using bearing you automatically generate a fake backsight point that by default is due north of the occupy station and 100 m. away. Remember to delete PT#2 from any analysis you do of the survey points. And again store the information.

IIIf. Zeroing Pentax on initial backsight

Now go up to the Pentax unit and turn it on. It will beep, give you a reading from 3 - 12 of the power level, and then it will blank the screen waiting for you to rotate the lens up to zenith and back down so it knows which way up is. It will then automatically "zero set" in the direction it is facing. This is the wrong direction. Sight the instrument in onto your prism-person who should now be standing with the prism on top of the backsight and holding the rod vertically so that the rod's leveling bubble is centered. When you are exactly centered on the prism, hit the "0 SET" button on the Pentax. Hit it again while it is beeping to zero set it (otherwise it assumes you hit that key by mistake). Your horizontal angle reading should be 359.59.59 or else 0.00.00.

A double check on the vertical 0 set is to flip the lens unit over and rotate it back 180 degrees and resight on the backsight. The measurement readings should read -90 degrees for the vertical angle and 180 degrees on the horizontal angle.

Now just to make sure everything is working, hit the "MEAS" key on the Pentax and if still centered on the prism, and asterisk should appear in the upper left of the Pentax screen showing that it is getting a signal back after bouncing the laser off the prism. It should then come back with distance and angle measurements.

From here on out all your commands to measure or anything will be sent from the SC-5, with you only rotating and sighting the Pentax on the prism as it moves point to point.

IV. Conducting the Survey

Most of your survey shots will probably be scattered topo points, called "side shots" because after shooting them you do not move the total station to that position, which is what you do when traversing. More on this distinction is a moment.

This is the Foresight screen:

IVa. Rod (prism) height

As stated before, the HT on the Foresight screen is ROD HEIGHT and is the height of the prism above the ground, gotten by reading the marks off the side of the stadia rod (no corrections needed).

IVb. Sighting in on the prism and measuring distances/angles

On top of the Pentax sighting unit (below the battery pack) is a small tube which shows an upward pointed triange "sight" when looked through. Use this to point the unit in roughly the right place. Then use the fine horizontal and vertical adjustment knobs to move the prism into the crosshairs visible through the eyepiece. The horizontal and vertical fine adjustments only work if the outer knob on each adjustment is "locked" or turned til it's tight so you can not rotate the unit by hand. Loosen the knobs before rotating the unit to shoot a new point. If you find that it is impossible to "lock" the adjustments so the unit won't turn, try tightening the lower clamp screw on the ring just above the 3 leveling screws.

To measure a point and send the information to the SC-5, hit [F2] measure on the datalogger from the FORESIGHT. The logger will prompt you to edit the description first and hit return. JUST HIT RETURN AFTER HITTING F[2]!!! You can edit the description before or after the measurement, but the SC-5 will not accept any reading from the Pentax as long as it is waiting for you to type in a description. This is one of a couple of really annoying design "flaws".

Once the point is measured, and the numbers from the measurement show up on the SC-5 screen, you can finish editing the code, description, and height, and then store the point.

IVc. Storing the location: "Sideshots" vs. "Traverses"

To store data for a point once everything is complete, normally just hit [F1] SIDE and a new FORESIGHT screen with a new pt. # will show up. And then shoot another point.

If you need to move the Pentax unit to another location to continue the survey, you are going to TRAVERSE to a new occupy station. Measure the point you will move to as before, but hit [F5] TRAV instead. This will give you an OCCUPY STATION screen with the pt # of the previous measured shot. Give the new Instrument height, and make sure that the backsight is to the pt # of the station you just left (#1 if this is the first traverse point after the initial set up). And continue.

IVd. Importance of written notes for error correction and troubleshooting

It is important to write down the point # of all your occupy stations and backsights so that you can reference them correctly when you traverse (the computer sometimes picks the WRONG backsight point when traversing). This'll cause all your points to be rotated to the wrong place, usually overlapping points you've already done and causing your topography to closely resemble an Eggo waffle. Also, if the rod height is wrong, your elevation will be wrong. This stuff can be fixed, some after-the-fact at home, some before you download it. Things like the wrong backsight can be cuaght if you save the raw SA files, but things like rod height and instrument height can only be caught and fixed if you take careful notes on your equipment setup measurements and changes in rod height, etc.

IVe. Graphing 2-D scatter of survey points on the SC-5

To graph scatter points while surveying, you must do something INCREDIBLY counterintuitive...you must switch the function key menu by hitting the OFF key (not shift-OFF), which is what that right-pointing arrow is suppose to mean. STUPID. But that's the way it goes. That will bring up another row menu that includes GRAPH (it also includes OFFSET if you need to measure to the center of things like tree trunks). Hit GRAPH [F5], and the screen will show your points. Pan around using cursor keys, zoom in with "I", zoom out with "O", quit with "ESC". If you set the graph's SETUP so that under FORMAT you select the following:
 

And under DATA the following:
 

Then what will graph will be a north arrow (either true or just your backsight) and all your points with a solid line connecting all your traverse points AND your initial backsight. If wanted you can also print up pt#s.

To graph outside of the FIELDWORK menu where you are actively collecting data, go to the main menu and into GRAPH.

***TO END THE DATA COLLECTION PART OF THE SURVEY AND GET OUT OF FIELDWORK MODE, HIT "ESC" TO GO BACK TO THE BACKSIGHT SCREEN, THEN OCCUPY STATION SCREEN, THEN THE MAIN MENU...BEING SURE TO HIT [F1] "YES" TO ALL QUESTIONS OF EXITING AND "NO" [F5] TO QUESTIONS OF DROPPING THE POINTS YOU JUST COLLECTED***

V. Correcting Survey Errors

Va. Changing code and description data

Go into the VIEW\EDIT section of the Main Menu, and select COORDINATEs. You can scroll down the list of points by point #, which also lists whether they were occupy stations, backsights, or sideshots, and edit a particular record by hitting EDIT [F1]. Then retype the code or description,

Vb. Changing instrument heights and rod heights

Go into the VIEW\EDIT section of the Main Menu, and select FIELD WORK. again you can scroll down and hit [F1] to EDIT a particular record, like rod height or backsight to pt# info or instrument heights.

Vc. Adjustments due to cumulative errors when traversing

Here there is a real problem and it is all MEMORY-DISK SPACE related. If you traverse a "closed" loop travers so that you end at the same occupy station you began with, you can run ADJUST so that the correction for the "drift" is spread through out the survey. BUT, this requires the datalogger to be less than half full since it has to create a second copy of the "adjusted" data in addition to that already stored, before it can erase the original data. So if you have collected a lot of points, right now you are out of luck unless you decide to do it yourself manually in a spreadsheet.

This would be solved if we could get a copy of the SA software that would run on the PC instead of the SC-5, but we don't, and I'm not sure if SA makes it for the PC (though they should)...

VI. Downloading the Survey Data back in the Lab.

Back in the lab, turn on the SC-5, and go to the FILE TRANSFER option of the Main Menu. Hook the SC-5 to the PC by using the cable from the "B" outlet of the switchbox and plugging it into the COM1 port on the SC-5. Turn the switchbox dial to "B". Go into Windows, and open up the applicatons window and click on "Download Total Station" icon.

VIa. Raw SA files

Inside the FILE TRANSFER option of the Main Menu, choose direction to be OUTPUT (use cursor keys). Change Format to be SA. Make sure protocol is KERMIT, not none. Hit [F1] GO. And the 5 raw SA files will be transfered to the c:\surveys\ directory.

VIb. coordinate-code-description files

Inside the FILE TRANSFER option of the Main Menu, choose direction to be OUTPUT (use cursor keys). Change Format to be either USER1 or USER2. Use the userdefined format 1 we've set up to output your coordinate file as a file containing each line to be a single point formated as follows:
 

Use USER2 for the same thing without CODE.

If you use their predefinied STD-Quotes, you will get the same info as USER2, but it will be Y,X,Z and not X,Y,Z.

Your coordinate file, regardless of format, will be named the same thing as your "job"s name, with no extention. Then copy the coordinate file to floppy disk to take with you (and the SA files too if you want to be safe).