And GPS

THE COMPASS

I WANT TO GO NW
First rotate the dial until it points the way you want to go
Then rotate your body until the north arrow points north
Walk in the direction of the arrow

AZIMUTHS AND BEARINGS

COMPASSES AND MAPS

COMPASS APP

MAGNETIC DECLINATION
The declination is given as e.g. "15 degrees east". When you look at the figure, you can pretend that plus is to the right, or east, and minus is to the left and west. So when something is more than zero you'll subtract to get it back to zero. And if it is less, you'll add. So in this case you'll subtract 15 degrees to the azimuth, by turning the compass housing, according to the numbers on the housing.

ADDITIONAL CONSIDERATIONS
Compasses work underground unlike GPS
Local magnetic sources can cause errors
Can take a ‘backsight’ on where you came from to confirm that it is 180 degrees off the traveled azimuth.
Compass and Pacing is a conclave competition.

PACING
1 pace equals the natural stride of both legs starting and ending with the same foot.
Can and should practice often – get consistent.
Recognize that is varies by slope, must compensate.

GUNTER’S CHAIN
 In 1620, the clergyman Edmund Gunter developed a method of surveying land accurately with low technology equipment, this was 66 feet long and from the practice of using his chain, the word transferred to the actual measured unit. His chain had 100 links , and the link is used as a subdivision of the chain as a unit of length.
 In countries influenced by English practice, land plans prepared before about 1960 associated with the sale of land usually have lengths marked in chains and links, and the areas of land parcels are indicated in acres . A rectangle of land one furlong (10 chains) in length and one chain in width has an area of one acre.
 It is sometimes suggested that this was a medieval parcel of land capable of being worked by one man and supporting one family.

GUNTER’S CHAIN

Conversions:
1 chain (1 ch) = 66 ft (100 links)
4 poles or rods = 1 chain
1 tally = 5 chains = 330 ft
20 chains = ¼ mile
80 chains = 1 mile
1 acre = 43560 square feet
640 acres = 1 section (square mile)
36 sections = 1 township
1 acre = 10 square chains
40 acres = 20 chains x 20 chains
1 section = 80 chains x 80 chains
CHAINS

‘THROWING’ THE CHAIN

HIPCHAIN

METES AND BOUNDS DEED SURVEYS
 Metes = distance to ‘turn’ of boundary
 Bounds = direction
 Historically the original 13 colonies and their derivative states
(from the British surveying)
 Problems:
 Described boundary points often change
 Doesn’t work well in homogenous landscapes
 Corrected for declination?
 Year surveyed?

PUBLIC LAND SURVEY SYSTEM

PLSS TERMS dimensions
(miles) (mile 2 ) area
(acres) (m 2 ) (km 2 ) notes
Quadrangle 24 by 24 576 368,640
Township 6 by 6 36 23,040
Section
Half-section
1 by 1
1 by 1 ⁄
2
1
1
⁄
2
640
320 1,294,994
Quarter-section 1 ⁄
2 by 1 ⁄
2
1 ⁄
4
160 647,497
1,492 Usually 16 townships
93 Usually 36 sections
2.6
1.3
80 323,749 Half of quarter-section 1 ⁄
2 by 1 ⁄
4
1 ⁄
8
Quarter of quarter-section 1 ⁄
4 by 1 ⁄
4
1 ⁄
16
40 161,874

 Antenna
 Receiver
 Base map
 Record tracks, waypoints, distance
GPS


Global

Positioning

System


Department of Defense developed for navigation

Standard positioning service (public uses)

Precise positioning service

Launches began in 1970s

Full operational capability in mid 1990s

‘Selective Availability’ turned off 2000

Degraded accuracy to ~100m


Space segment – (satellite life = 10 years)

Several generations of satellites in use now

Control segment

User segment






24 satellites with spares in 6 orbital planes (4 in each)
~12,500 miles elevation
55 degree inclination
Each one circles
Earth every 12 hours
(7000 mph)
At least 4 visible at every point on the earth at all times


Master Control
Station (MCS) in
Colorado

5 Monitoring stations

Ground control stations (Ground
Antennas)

Unmanned

Enable MCS to control the satellites


Antenna

Receiver

Base map

Record tracks, waypoints, distance


Triangulation

Need D+1 satellites to determine position =
2D needs 3 satellites, etc


Dual frequency mode of more advanced receivers corrects

SOURCES OF ERROR - SATELLITE GEOMETRY
Geometric Dilution of Precision (GDOP) – the higher the value, the poorer the measurement (very good = <4, bad = >6)


Recreation grade = ~8m

Mapping grade = <1m

Survey grade = several cm

Accuracy versus Precision

Accurate measurement versus true location

Precise measurements close to each other


Most GPS in cell phones do not use satellites.

They triangulate via cell towers whose locations are precisely known.

The signal is not line-of-sight so they can work indoors.

Does not work where limited cell service exists
(like many forests).

Accuracy varies (< 10 m to ~100 m).

AREA DETERMINATION
 Compass and Pacing
 Map with dot grid
 Cut and weigh method
 GPS
 ArcGIS or similar mapping software