Fld Methods Session 3 - Trig, topo maps, plane table, etc.
Questions on the first part of Bayonne Park?
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What do you like? And what not?
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Strengths/weaknesses of pace/compass?
I.
Step up the level of accuracy (and slow self down…)
Now we move to specific data, starting with points.
A. coord systems …. How locate points? Essentially in xyz space.
Z (elevation)
X axis (E-W)
Y axis (N-S)
Global view….
Latitude
Longitude
Fld Mth sess 3 p.2
Diff coord systems depending upon projection system, etc.
Exercise - compare lat lon to x,y on topo map
B. ways to collect point data - all sorts
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points (outcrops or elevations) on a traverse
GPS receiver
Laser total station
Alidade on a plane table
This last is what we'll use next, with GPS.
C. little bit of trig refresher
it's all about right triangles….
Slope distance (hypotenuse)
(what you walk)
Vertical
distance
(opposite)

Horizontal distance (what you measure on the map)
(adjacent)
Sin  = opp/hyp
Cos  = adj/hyp
Tan  = opp/adj
Fld Mth sess 3 p.3
D. basics of the Alidade - Compton ch 8 excellent summary
first, recognize this unit is outdated, no longer in use…..supplanted years ago by
electronic total station.
WHY?
TS faster, easier to use - sends out a laser beam, it bounces back from a mirror.
Computer in TS calcs distance, it keeps track of its "vertical angle". So you
automatically get a bearing, a distance, and a z value in a fraction of time you
calculate and plot these data with the Alidade.
Modern GPS getting close to supplanting total station, but not yet accurate
enough.
So why are we stuck with alidade?
 It's what we own.
 It's a good teaching device.
 It's still pretty accurate.
 Trig is trig…..all the same basics, just diff tools.
define some terms, look for the key measurements
go to Compton ch 8 (new) p. 135, play with ones in the lab.
How do they work?
In conjunction with a plane table, rod, and tripod, it's a self-contained mapping
system. You sight on a rod, draw a bearing from the plane table to the rod,
calculate the distance and elevation of that point, and have the rod person move
on to the next station. When you've done all you can do from your current plane
table position, you sight to your next one with your rod man, then set up at your
new location.
You can imagine how much faster this would go with a total station with a
memory that is storing coordinates as you go….
The GOOD news - this is probably the first and last time you'll use one of these
antiques…..
The BAD news - you have to do it, and you'll never forget it!!!!
Fld Mth sess 3, p.4
We'll worry about plane table set up later, but for now, we want to get the data
collection sequence down…how to put togethet a data "spreadsheet", what is our
procedure, what are we doing and why, etc….
E. sec 8-4, stadia msrmts
terminology - there are 3 horiz lines (hairs) in your sight and they ALL are
important.
The outer ones are the stadia hairs, the middle one is the cross hair. The outer
ones are key for getting horiz distance, the middle one is key for the vertical
distance. Remember, you are essentially sighting along a hypotenuse, but you
need to emerge with an opposite (vert) msrmt and an adjacent (horiz) msrmt
CH
Stadia rod
Ht Inst = 514.5
Grd elevation = 510.2
Key concepts:
 Stadia intercept (SI) is the distance on the rod between upper and lower
hairs. The alidade optics are set such that 100 x SI = slope ("stadia")
distance.
Conversion to horiz distance :
 HD = (SI x 100) - (H factor x SI) , where H factor is read off the Beaman scale
This is the horiz distance from the rod to the center support of the alidade
In the field, with a predetermined scale in mind, you can plot the bearing and
horiz distance right away….
Fld Mth sess 3, p.5
Vertical distance considerably easier to screw up, trying to keep track of
instrument elevation and the crosshair elevation.
The vert distance between the crosshair and the instr elev. is pretty simple:

VD = (V-50) x SI ; V factor is read from the Beaman scale - when V = 50,
alidade is horizontal
To get the vertical distance from the alidade to the ground elevation of the rod,
you always subtract the crosshair reading from the above equation:

VD = [(V-50) x SI ] - CH
Why do you always subtract the CH? Because that number is always ABOVE the
ground elevation at the base of the rod, no matter if you're siting uphill or downhill
Caveat to the above formula - the algebra works fine for foresights, but not for
backsights…..Freeman book has some better explanations and pics than
Compton
Freeman has a good formula that can be used for foresights and backsights
Foresight: know elev instrument, want Elev Rod pt (GL at the rod base)
Backsight: know GL at the rod, want inst elev at new location
Where "GL" means Ground Level or Ground Elevation
One formula works, just shift the terms around from one side to the other
Foresight:
Elev RP = Elev Inst + [(V-50) x SI] - CH
(where ERP means GL at the rod point)
Backsight:
Elev RP - [(V-50) x SI] + CH = Elev Inst
Fld Mth sess 3, p.6
Let's get some practice in…..set up the alidade, take some horiz and vertical
readings…..
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Set up the tripod
Attach Plane table
Level the Plane table using circle bubble on alidade, tighten upper Johnson
head screw
Orient the plane table by lining up edge of table with true North (alidade has
needle that will give you magnetic north, use your Brunton to spot the 2 lines
on your paper
Level the alidade barrel with the striding level
Level the Beaman scale with vernier bubble (V should be at 50 when horiz)
Now practice an exercise in the hall - work in teams, but give me your
calculations and answers individually (everyone needs to do the math…)
Each team sets up a table over a blue square, will sight to rod at end of hall
1. GIVEN: your instrument station GL is 50 feet above sea level.
a) Find your instrument elevation
b) find horiz distance to rod
c) find ERP (Grd Elev at rod)
to make this a little realistic, do this twice: once with the alidade pointing a little
up, and once with the alidade pointing down, to get the barrel away from
horizontal
2. GIVEN: rod GL is 50 feet above sea level
a) use the formula for a backsight in order to determine your instrument elevation
at your inst station. Use either one of your data sets from #1 to work this problem
follow the formulas…use spdsheet I created if you wish…
DUE on SAT Morning at the Park……