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Monday, February 17, 2014

Basics of Surveying



Surveying Basics

1.0 Definitions
Surveying – the art of measuring distance and angles on or near the surface of the earth
Plane surveying – the land surface is considered to be a plane for all X and Y dimensions,
and all Z dimensions (height) are referenced to this plane. Most engineering surveys are
plane surveys, those that cover long distances may need to correct for the earth’s
curvature.

2.0 Equipment
notebook – record all your data and sketches
transit – used to establish straight lines and measure horizontal and vertical angles
tape – measures distances
level rod – measures distances in elevation
total station – electronic measuring instrument that records and processes field data
prism rod – locates the points of interest and reflect laser back to total station for angle
and distance measurements
engineer’s tape – measures the height of the instrument (transit or total station)
survey nails – locate points of interest for future use
two-way radios – keeps contact between the instrument person and the rod person
2.1 Notebook
One of the most important aspects of surveying is taking accurate, neat, legible, and
complete field notes. The degree of completeness comes from practice. Details that
seem obvious in the field may be obscure back in the office a couple weeks later.
Sketches of your surveyed area will also aid in comprehension and ordering of the data.
These sketches do not need to be to scale. Do not crowd your notes.
At a minimum, ALWAYS record the instrument height, rod height, occupying point,
backsight point, and datapoint ranges in your notebook. Then, if there was an error in
inputting the correct data into your total station, you have a double check. A description
and sketch of each surveyed point are very useful as well. Figure 2.1 shows the basics
that should be included in all field notebooks.

Figure 2.1. Diagram of data to include in field notebook.

2.2 Level Transits

Figure 2.2. Schematic of the use of a Level Transit

For the example shown in Figure 2.2, we can determine the elevation difference between
Points 1 and 2 as such:
Reading on Level Rod 1 from horizontal line of sight = 2.63 ft
Reading on Level Rod 2 from horizontal line of sight = 7.21 ft
Elevation Difference = 4.58 ft
The distance between the points can be determined with a measuring tape.
If elevation at the instrument is known:
Actual Elevation at Point 2 = Elev at instrument + HI + LR1 Reading – LR2 Reading

If elevation at Point 1 (backsight) is known:
Actual Elevation at Point 2 = Elev at Pt 1 + LR1 Reading – LR2 Reading

2.3 Total Stations
A total station records and processes all the data collected in the field. It measures
distances and angles by use of a laser shot from the instrument and reflecting prism on
the survey rod. A total station is simply a computer. It can only know what you input.
When you orient the total station with a 3-D location and an angle at each setup, it will
reference all ensuing point data to that.
2.3.1 Location
If there is a benchmark available that has known XYZ coordinates, then great, use that as
a setup or backsight point. Most sites that EWB work in will not have benchmarks. You
have two other options, and the choice depends on the level of detail needed in your
survey. If precise detail is needed, bring an accurate GPS system with you to locate your
position. If relative detail is needed, then enter any value for your location and reference
everything to that point, i.e. label your northing as 5000, easting as 10,000, and elevation
as 1000. If the precise coordinates are not necessary, this latter option may be easier to
perform in the field.

2.3.2 Angle
Just giving your total station a location lets it know where it is, but it doesn’t know where
it’s looking. So, you will have to orient the instrument as well. The options for doing
this again depend on the level of precision needed for the project. For accuracy and
precision, you can either bring a compass that tells you which direction is true north, or
you can shoot a second point that has known XYZ coordinates. Again, this latter one will
be very rare in most EWB locations. If you bring a compass, then station your prism rod
in the direction of true north and shoot with the total station. The instrument will then
know where it is and where it is looking.
If precision is not necessary, you can make an educated guess as the direction of north
(without the aid of a compass) and shoot the prism rod. This will at least orient your
instrument and the rest of your points will be referenced to this location and direction.

2.4 Leveling
For each setup, the survey instrument must be level. The locations and orientation
established above assume a level instrument. The basic procedure for tripod and
instrument leveling is provided below:
1. Center tripod over desired point at an appropriate height and make the base fairly level
2. Attach the instrument to the base screw (Note: Do not let go of instrument until the bottom screw has been attached)
3. Turn on the instrument if using a total station
4. Turn on the laser level or attach plumb
5. Maneuver the tripod to center the plumb on the control point
6. Coarse-level the instrument by adjust the tripod legs individually until the level
bubble is mostly centered (Note: this adjustment will only minimally adjust the location of the plumb)
7. To fine-tune the leveling, orient the three black fine-tune level knobs (at base of instrument) as a triangle with a point facing away from you
a. Adjusting the two closest to you at the same time, turning them both inward will move the bubble to the left. Turning them both outwards will move the bubble to the right.
b. Adjusting the knob away from you, turning it counter-clockwise will move the bubble away from you. Turning it clockwise will move it towards you.
8. After instrument is level, check location of plumb. If you are within a couple of inches, you may move the instrument by half-unscrewing the bottom screw and sliding the instrument into place. If you are too far off, then you will have to move the whole tripod into position.
9. Repeat until the instrument is level and over the correct point.

2.5 Viewscope Orientation
If you are using a total station, sometimes it may not be obvious as which way is up for
the survey gun. The correct orientation has the horizontal fine-tune knob on the lower
right and the vertical fine-tune knob on the upper left as you are sighting through the
instrument. Running the instrument upside-down will invalidate your data.

2.6 Moving the Instrument
When you shoot a point to use as your next location, place a nail or some other relatively
stable marker in the ground. Move the instrument to the new nail. When setting up the
total station, tell the instrument which point you are on. Since you have already shot this
point, it knows its location. Backsight to another control point, and the instrument will
now be correctly oriented and ready to continue on. Be sure to check your error after the
backsight. If it is large, there might have been a mistake in some previous measurement
that you’ll need to double check.

3.0 Point Descriptions
Be consistent with point descriptions. Label all your control points the same, i.e. do not
label them as “control1”, “control2”, etc. Label them all as “control” (or some variation
thereof) and let the point ID number be the differentiating factor between them. The
importance of these distinctions will become apparent if you are using a CAD program
afterwards to draw up your survey points, since each point descriptions will become its
own layer in the program.

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