Line and polygon features can be created via on-screen digitizing.

Transcription

1 This module explains how GPS works, sources of error, and error correction using real time or post processing differential correction. Cost and accuracy of different grades of GPS units are also part of this module. We will talk about how you can add your GPS data into a GIS. Line and polygon features can be created via on-screen digitizing. 1

2 GPS requires three components: Direct connection to at least 4 NAVSTAR satellites, a GPS receiver, and location on or near (airplanes) the earth s surface. 2

3 GPS satellite constellation 3

4 GPS receivers calculate their position by measuring the time it takes for the signal to travel from the satellites to the receiver. Satellite signals require a direct line to the GPS receiver. Signals can not go through water, soil, walls or others. 4

5 The time it takes from the GPS signal to travel from the satellite to the receiver is measured and then converted to distance by multiplying with the speed of light. Distance measurements form 4 satellites is necessary to compute a 3-d position (including a reasonably accurate measure of elevation). A lower accuracy 2-D position can be computed from distance measurements from three satellites. Differential correction can be applied to the signal to correct for atmospheric and other errors. We will talk more about differential correction later in this lecture. 5

6 Many contributing factors add to the error associated with acquisition of a GPS location. In the next few slides you will explore the sources and influence of the different errors on the total GPS error. The last bullet in the slide addresses the error introduced by Selective Availability. Selective Availability was a ~100 m error introduced to the GPS signal by the Department of Defense. Selective Availability was turned off in May 2000, and is currently not affecting the accuracy of GPS receivers. 6

7 The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space. 7

8 A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user. 8

9 PDOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites are used for the measurement. Different satellite geometries affect the errors. A greater angle between the satellites lowers the PDOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement configuration. PDOP under good conditions is below 6. The accuracy specification for a high end GPS unit is only valid when the PDOP is below a specified value (usually 6). 9

10 Differential GPS (DGPS) incorporates a series of techniques to account for some of the GPS errors. Differential GPS can be applied on the fly (real-time differential GPS) or post-processing differential correction (correction applied to the GPS data after it has been collected). 10

11 Both real-time and post processing differential correction involves two receivers, one that is stationary and one that is roving. The stationary unit collects data continuously and the location for this unit is therefore very accurate. The roving unit is carried by the navigator (mapper, surveyer, recreationist.). As you learned in previous slides GPS errors are influenced by many factors, but receivers within 100 km usually have about the same errors. The stationary receiver ate a precisely known location compares the current reading with the known location and can tereby compute a correction factor. In real-time differential GPS this correction factor is transmitted to the roving unit. The user of the roving unit reads the corrected GPS coordinates directly on the GPS display. In post-processing differential correction the correction factors are posted in a database, many are available via the Internet. The GPS data from the roving unit is downloaded to a computer and corrected using the information from the stationary unit (base-station). Post-processing can currently not be applied to recreational grade units. 11

12 Base station are located all over the country, commonly managed by the Forest Service, Bureau of Land Management, US Geological Survey, Department of Transportation or the US Coast Guard. Base station data used in postprocessing differential correction can accessed at: WAAS is a system of 2 satellites and 25 ground stations that transmit a realtime differential correction signal to GPS units that are WAAS enabled. There are also two commercial satellite differential service providers, Thales Survey LandStar (formerly Racal LandStar) and OmniSTAR Inc.. These companies use a control hub where reference station data is checked, formatted, and uploaded to a geostationary satellite for broadcasting to subscribers for an annual fee. A beacon can be attached to many GPS units for reception of the real-time differential signal. One example is the Beacon on a Belt from Trimble: If you cannot receive a real-time differential correction signal, post-processing is another way to achieve differentially corrected data for higher end mapping units (not available for recreational units). 12

13 WAAS is a network of 25 ground stations and 2 geostationary satellites put up by the Federal Aviation Administration. The system broadcasts a real-time differential correction signal at no annual fee or registration requirements all you need is a WAAS enabled GPS unit. WAAS is not received everywhere or all the time. For GPS you can expect an improved accuracy of ~7 meters during the times the GPS unit receives the WAAS signal. Read more about WAAS: 13

14 Base station data for post-processing differential correction can be accessed at: Read more about differential correction at: 14

15 New GPS technology allows the user to load aerial photographs or topographic maps into the GPS unit for display during the GPS data collection. The current GPS locations is displayed on top of this backdrop of photo or map. Software also exists (for example ArcPad from ESRI) that allows the user to create features via on-screen digitizing on the GPS unit or Pocket PC. Read more at: 15

16 If you are going to import your collected GPS data to a GIS it is advantageous to record the GPS locations in the same coordinate system as your GIS layers are in. The default setting on most GPS units is to collect in Geographic coordinates (Degrees, Minutes and Seconds) and the WGS84 datum. If you want to record GPS locations in a different coordinate system and datum you can adjust the settings under SETUP on the GPS unit. It is important to coordinate data collection within a field crew such that all members of the crew records data in the same coordinate system and datum. As we learned in the Map Projection section of this course the map projection tools in ArcToolbox can be applied to convert data from one coordinate system and datum to another. This is however contingent upon the fact that the original coordinate system and datum for the collected data is known!! Units can be set to collect in Degrees, Minutes, Seconds or degrees and decimal minutes or in decimal degrees along with other coordinate systems such as UTM (Universal Transverse Mercator). Format for degrees, minutes and seconds: DD MM SS.ss Format for degrees and decimal minutes: DD MM.mmm Format for decimal degrees: DD.ddddddd If data is collected in decimal degrees it is important to record at least 6 decimals. 16

17 Map coordinates are present on most maps. This 7.5 minute USGS quadrangle map shows coordinates in both degrees&minutes and in the UTM coordinate system. The map datum is also available in the map information. 17

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19 ArcGIS requires input data to be in decimal degrees or in projected coordinates (UTM for example). You can convert data expressed as DMS (degrees, minutes, seconds) to DD (decimal degrees). There are 60 minutes in a degree and 60 seconds in a minute, and therefore 3600 seconds in a degree. 19

20 You can convert DMS data to DD in Excel. Remember that longitudes in the western hemisphere are negative as are latitudes in the southern hemisphere. 20

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23 Right click on the table that contains the GPS data and select Display XY Data 23

24 Specify what column contains the X coordinate and which contains the Y coordinate. If you click on Edit, you can specify the coordinate system and datum for your GPS points. Let s say your data is in latitude and longitude coordinates. Is longitude the X or Y coordinate?? 24

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29 You can update the area in the attribute table using Xtools or add a column called area in ArcGIS and calculate geometry for this new field. You can add additional columns and label each polygon. 29

30 In the layout view in ArcMap you can add a coordinate grid over the map. These maps are particularly useful for field work. 30