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The GPS Offset Protocol

The GPS Offset Protocol. Under Conditions of heavy canopy, some GPS units may fail to acquire enough satellites to make a reading possible. In this situation, the “Offset” protocol is used. For the Offset protocol, we use the fact that every 0.0001 of latitude is equal to ~ 11 m. o.

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The GPS Offset Protocol

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  1. The GPS Offset Protocol

  2. Under Conditions of heavy canopy, some GPS units may fail to acquire enough satellites to make a reading possible.

  3. In this situation, the “Offset” protocol is used. For the Offset protocol, we use the fact that every 0.0001 of latitude is equal to ~ 11 m o o 0.0001 = 11 m

  4. Every unit change in this place represents a change in position of 11 m

  5. To use the Offset protocol, we move either North or South a measured distance to a clear area

  6. In this clear area we acquire a GPS reading o N 38.8760 W 094.7990 o South 77 m to previous position

  7. Since we have moved only North or South, our Longitude reading is the same. We must correct only our Latitude reading. Why Only Latitude?

  8. Lines of latitude are parallel and everywhere on the Earth 1 of latitude is approx. =111 km. o

  9. Lines of Latitude are parallel and everywhere on the Earth 1 of latitude is approx. =111 km. o Lines of longitude converge at the Poles. 1 of Longitude varies from approx. 111 km at the Equator to 0 km at the Poles. o

  10. Since we have moved only North or South, our Longitude reading is the same. We must correct only our Latitude reading. In this example, we moved 77 m a. We divide our distance by 11 m/.0001 degree b. 77 m / 11 m = 7 c.Our correction to latitude is then 0.0007 degrees This is the change in the 0.0001 degree place in our GPS reading. Since we moved North, this change is subtracted from our new reading.

  11. Doing the Correction o 38.8760 Latitude at Offset Position - o .0007 Correction Subtracted o 38.8753 Corrected Latitude o GLOBE Data Entry would be 38.8753

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