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.
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.Edmund Gunterlinks 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.acresfurlong 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
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? METES AND BOUNDS DEED SURVEYS
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 Triangulation Need D+1 satellites to determine position = 2D needs 3 satellites, etc
Dual frequency mode of more advanced receivers corrects
Geometric Dilution of Precision (GDOP) – the higher the value, the poorer the measurement (very good = 6)
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).
Compass and Pacing Map with dot grid Cut and weigh method GPS ArcGIS or similar mapping software AREA DETERMINATION
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