4.19.03 How to Use GPS in CLUP Data Preparation

Tue, 11/27/2007 - 11:10
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these satellites as reference points to calculate positions accurate to a matter of meters or up to centimeter level in advanced forms of GPS receivers. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical, making the technology accessible to virtually everyone. These days GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, and of course into laptop computers. Because of the great reduction in price (same price as a mid range cellular phone), GPS surveying is a practical way of getting primary data for the CLUP GIS. This chapter will now show how GPS works, and presents tips that may not be found in the GPS user manual. GPS Survey Know your receiver A GPS receiver is a new device for many people who should familiarize themselves with the device. This information can save time, resolve problems in operation, and prevent accidental data loss. Read the manual. Know the specifications of the GPS model, set-up, functionalities, how to use, and the limitations of the gadget. Find out what operations would result in data loss (reset, delete, clear track records, etc), in order to prevent accidental data loss. Take the time to practice using the receiver before conducting an actual survey. Practice set-up procedures, data collection and data deletion. Observe signal strength and level of accuracy for different weather conditions and locations. Learn to clear all captured data (tracks, waypoint, routes). Observe what happens to the other data if one set is erased (e.g. would erasing all waypoints have an effect on the route). Observe battery life of the GPS receiver. This is important in planning the survey and in case there are no available spare batteries for the unit. Plan the survey GPS survey will consume time and resources (gasoline for the vehicle, manpower). Here are some things to do in planning the survey. Prepare a map of the area to be surveyed and have it printed in a size that can be easy to write on and read while in a moving car or outside the field (A3 or A4). Find source persons for the places to be surveyed if nobody in the survey team is familiar with the places. Plan the survey routes on the map. If working in teams, assign which areas to cover and alternate areas to survey in case the survey is completed early or if problems arise for primary areas. Consider hazards and risks that may be encountered in the survey areas (flash flood areas, road conditions, insurgents, bulwarks of rival political parties, etc.) to avoid delays and prevent any untoward incidents from happening. Plan who to bring with the team, what type of vehicle to use in certain areas (model, marked or unmarked, etc. ) to avoid physical (rough road) and social (presence of insurgents or areas controlled by rival political parties) constraints. Bring along a resource person for the areas during the survey if possible. Assign roles to the members of the survey team (GPS operator, recorder, photographer). Prepare materials before conducting the Survey Prepare extra maps of the route plan if more than one team is organized. Prepare data sheets for the GPS readings. This serves as the back up for the data and a much easier way to write down notes. Use a clipboard folder for writing data in the field. An example of a GPS data capture sheet is found in Chapter 6.04. Bring spare batteries, if available; otherwise make sure to fully charge the battery before going to the field. Bring a digital camera if available and take pictures of the areas being surveyed. Before leaving for the survey Check the weather if GPS survey is possible. Check if all materials are okay and ready to go to avoid any delays. Initialize and set-up the instrument before going out to the field Turn on the GPS receiver and apply appropriate settings. Here are some of the settings that should be applied that would be common to most receivers. Datum : WGS84 Units : Metric Bearing : True Latitude/Longitude Units: DD:MM:SS.S (Degrees-Minutes-Seconds) Find an open space with a good view of the sky and wait for the GPS to set-up. Once the GPS is set-up, take some test readings. Take note of the accuracy of the test readings. This will serve as an estimate for the next readings. Place receiver in the dashboard while inside the vehicle to prevent signal loss thereby eliminating the need to set-up the receiver again. Getting the readings
  1. Locate an open area on the area to be surveyed. If the area is a small building (less than or equal to two floors), find an area near the structure that will still have a good view of the sky.
  2. Get GPS readings on the area and observe the accuracy. Wait for the accuracy to stabilize, at least to a meter level, and take note of the highest accuracy level possible.
  3. Save the reading when the accuracy is within the highest stable reading possible.
  4. If accuracy reading fluctuates and is unstable, try to find a new area and repeat the readings.
  5. If a different area continues to give unstable reading even for a long period of time, save the data with the highest observed accuracy.
  6. Record the readings on the data sheet and the point number recorded in the GPS.
  7. Take pictures of the area using a digital camera and record the photo number/s in the data sheet for the corresponding point.
Notes:
  1. Take readings from the same area per feature type. (e.g, flagpole for all schools, front of building for all Barangay Halls).
  2. Stay away from tall buildings and sources of strong electrical current or interference (e.g, transmission lines, substations, electrical generators, cell sites);
  3. Track reading is a feature common to GPS receivers today. It records the route of the survey as points that can also be downloaded into the computer. Turn this feature on whenever available. Clear track readings in the receiver whenever starting a new day for survey but before doing so, make sure it is already downloaded and saved in a computer. Take note that track readings (called ‘track logs’ in some models) is different from point readings (‘waypoints’ in some GPS models) to prevent any accidental erasesure of other data stored;
  4. Set-up the GPS using the above procedure whenever it is turned off (e.g. lunch break);
  5. Download the readings (points and tracks) to a computer after a day’s work. Next page contains a form that can be used for the data capturing in the field. In Chapter 6.04 the Excel file is found.
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