Plotting GPS derived Coordinates on a Map

Transcription

1 Plotting GPS derived Coordinates on a Map Introduction to GPS Global Positioning Systems (GPS) is a technology whereby a constellation of satellites sends out signals which can be received by a GPS receiver (e.g. a hand held Garmin GPS unit, Garmin Nuvi, Tom Tom, etc.). Those signals can be converted to distance measurements which can be used in triangulation algorithms to determine the position of the GPS receiver. This is all done automatically in the receiver. With contemporary units, the user typically sees the current position indicated by an icon on some sort of base map displayed on an LCD screen. Positions can be saved in the unit or simply the coordinates be displayed on the screen. Positional Coordinates: Latitude/Longitude If we convert meridians and parallels into a Cartesian coordinate system, we can use Latitude (parallels) and Longitude (meridians) to display our position on a 2 D (i.e. flat) map.

2 Latitude meridians and Longitude parallels can have direction relative to datum. The position relative to the datum is given by an East or West notation for Longitude and a North or South notation for Latitudes. For example, in Gainesville our location is W82.34 o, N29.67 o. To convert this to a Cartesian coordinate system, W and S are negative. Therefore, our position is ( 82.34, 29.67). Lat/Lon coordinates can be displayed in three ways: degrees, minutes, seconds: 82 o 42' 21" degrees, decimal minutes: 82 o 42.35' decimal degrees : o Note: 1 degree = 60 minutes Also: 1 minute = 1 nautical mile = 1852 m 1 minute = 60 seconds 1 degree = 3,600 seconds Thus, W82 o 42' 21" = ( / /3600) = o

3 Other Coordinate Systems: Useful when integrating data into a Geographic Information System (GIS) There are many ways in which to stretch or contort the spherical surface of the earth on to a 2 D map. Visualize shining a light through the earth and the subsequent shadow that is projected onto a geometric shape (e.g. a cone or cylinder). These are called projections. The nature of the shape and how the shape is oriented relative to the earth controls which parts of the earth's surface are distorted and which are not. Therefore, each projection has its optimal use or application. For instance, the Albers Equal Area projection is a conical projection that minimizes distortion in the N S and E W direction. Therefore, it is the projection of choice for agencies such as the Florida Department of Environmental Protection, who must deal with state wide data sets in the E W oriented panhandle and the N S oriented peninsula. Most GIS data will be in a particular projection and you may or may not be able to configure your GPS to match that. Math can be used to move from any system to any system.

4 Accuracy and Precision of GPS and Coordinates Because many projection systems exist, it is acceptable to simply record the Lat/Lon coordinates and allow the end user to reproject into an appropriate projection. It is NOT acceptable, however, to report units with an inappropriate amount of decimal places. Care must be given to carefully match the number of decimal places to the accuracy of the GPS and the desired precision of the data. For example: 84 o = o How many decimal places do we need to report? Suppose you have a survey grade GPS with an accuracy of 1 cm: 1 decimal place = 1/10 degree = 111,120 / 10 = 11,112 m 2 decimal places = 1/100 degree = 111,120 / 100 = 1,111.2 m 3 decimal places = 111,120 / 1,000 = m 4 decimal places = 111,120 / 10,000 = m 5 decimal places = 111,120 / 100,000 = m = cm 6 decimal places = 111,120 / 1,000,000 = m = cm 7 decimal places = 111,120 / 10,000,000 = m = cm 8 decimal places = 111,120 / 100,000,000 = m = cm We must report 8 decimal places to match the precision of reporting to the accuracy of the GPS unit. In practice, most positional coordinates are determined using either a mapping grade GPS (e.g. Trimble Geo XT) which deliver m accuracy on instantaneous measurements or on consumer grade GPS units (e.g. Garmin GPS Map 76C) which deliver 1 3 m accuracy. Most units now allow for point averaging, where you can leave the GPS receiver static at location. The unit will automatically record a position each second and then average those together to give a more accurate position. Finally, the mapping and survey grade units allow for post processing of GPS points to further improve accuracy by removing errors using correction logs downloaded from reference stations on land.

5 Regardless of GPS grade, instantaneous vs. static measurements, or post processing vs. real time, you must be aware of what your accuracy is so that you conform to whatever standards are appropriate for your work and so that you do not report too high or too low of a precision. For example, a typical consumer grade WAAS GPS will yield a 1 3 m accuracy. For that grade of GPS, reporting 6 decimal places will preserve the 1 m accuracy because our coordinates are precise to 11 cm. If we report 5 decimal places, we have a precision of 1.1 m which is just above our maximum accuracy. We certainly don't need more that 6 decimal places. WAAS = Wide Angle Augmentation System. It is a separate satellite that broadcasts correction factors for atmospheric and other typical phenomenon, thus allowing a consumer grade GPS unit to be 1 3 m accurate instead of m accurate. Most GPS units since 2000 are WAAS. Not all units have this feature activated by default. Post processing is done by accessing Continually Operating Reference Station (CORS) databases to retrieve atmospheric and other corrections that can be applied to your data to increase accuracy. Only mapping and survey grade GPS units allow you to do this. Consumer grade units don't record the additional information necessary to pair your readings with the database. Recording of Coordinates in the Field Using GPS You have three choices when you record a location in the field. The safest way is to use the GPS to report the coordinates on the screen, and you write those down into your field notes. This is good practice and could be required depending on your line of work. Another choice is to record those points internally to the GPS unit and then export them to your computer. You can do both as well. If you write down your coordinates in Degrees Minutes Seconds, then you can type them into a spread sheet and mathematically convert them to decimal degrees for input into an online mapping website. If you export them to a computer, you are likely intending to take them to a GIS environment, although there are some utilities that will export to an Excel file as well. Finally, when recording GPS coordinates, try not to stand over the unit or otherwise block the antenna. This obstructs the view of satellites and will reduce accuracy. Also, using next to a large building will block out some satellites and cause multipath error from the ones your GPS unit can see. Multipath is when a signal hits a tall building first and then is received by the GPS. It extends the travel distance and therefore increases the error. Finally, dense forest canopy cover can degrade signal quality and inflate error. Using a higher quality (i.e. more expensive) GPS in these situations can reduce the effect of these errors.

6 Some GPS units allow for offset so that you can remotely record GPS coordinates to avoid multipath near objects such as buildings. A third choice which is not widely implemented is to real time connect the GPS to a computer (e.g. laptop or PDA) and use software such as ESRI ArcPad or TeraSync to record points directly into the computer. This third option is not widely implemented because field ruggedized computing equipment is still pricey (e.g. $2000 for a field computer compared to $200 for a Garmin GPS unit).

7 Plotting GPS Coordinates Once you have GPS coordinates in decimal degrees format, you can input them into one of the many online mapping websites. Microsoft Live Search Maps is currently very user friendly and has some of the better looking aerial photography: For this website, if you want to plot a GPS location, you must input coordinates as: decimal latitude, decimal longitude (other sites may vary). Therefore, Gainesville is: 29.67, (shown in red arrow below) You can then click the Aerial (blue arrow above) to see the aerial photograph of your location.