Geographic Coordinates or longitude/latitude will vary depending on the assumed shape of the globe

Transcription

1 Geographic Coordinates or longitude/latitude will vary depending on the assumed shape of the globe 1

2 Latitude is the angle north or south (think ladder going north south) Longitude is the angle east or west (think longer as 360 degrees of longitude compared to only 180 degrees of latitude) 2

3 The earth can be approximated as a sphere with a radius of 6,370,997 meters. 3

4 There are many spheroids representing the planets.they are typically defined using parameters such as semimajor and semiminor axis distances in meters. Here are three common spheroids for the earth and one for Mars. 4

5 Spheroids are used by datums. A north american datum may represent the gravipotental earth surface well in north america, but not europe. 5

6 The three spheroids most commonly used in North America are the Clarke spheroid of 1866, and two very similar spheroids (GRS1980, WGS84) 6

7 Longitude and latitude should be expressed with reference to the horizontal datum used. 7

8 Note that the NAD83 and WGS84 datums use spheroids that are almost identical, so for most GIS applications the two datums are very similar. 8

9 The difference between the same longitude/latitude value in NAD27 versus NAD83 is about 400 feet near Fairbanks! 9

10 GIS typically expect GCS coordinates in decimal degrees. 10

11 1) GIS expect coordinates X,Y and 2) X should be negative since longitude in negative west of the prime meridian 11

12 You should always enter at least 5 digits to the right of the decimal. 12

13 Notice there is a substantial difference on the ground between and latitude (370 meters in the case) 13

14 The antenna of a GPS base station is located on the roof of this building. This is a Continuously Operating Reference station or CORS as part of a global network of GPS base stations. You can get informatioon about this GPS base station from

15 You can create a point layer from a table of GCS coordinates in decimal degrees using the Make XY Event Layer tool. 15

16 If you incorrectly specified the datum as NAD27 instead of NAD83, your position will be in error of about 125 meters! 16

17 There are hundreds of map projections because no one projection is distortion free when projecting the globe to a flat plane 17

18 All planar coordinate systems based on a projection originate from a specific Geographic Coordinate System 18

19 Projected parameters typically include terms like central meridian (north south line) or standard parallels (east west lines). For example, the UTM Zone 6 projection has a central meridian at -147 degrees, the Alaska Albers statewide projection has standard parallels at 55 and 65 degrees. 19

20 Azimuthal projections have one point where the scale factor is 1.0 Cylindrical and conic projections typically have one or more lines where the scale factor is

21 If a map projection touches the globe along at one line, it s case is tangent. All locations away from the tangent have a scale factor >

22 Here is an example of a secant case where two locations touch the globe. You can have a scale factor < 1.0 and >1.0 with a secant case map projection 22

23 Projected coordinate systems are most commonly used in GIS instead of geographic coordinates. 23

24 Create a graticule of longitude/latitude lines. 24

25 Data displayed in geographic coordinates (NAD83 datum) Notice that the border of Alaska north of St. Elias area is a constant 141 degrees west. 25

26 The National Park Service and US Fish and Wildlife service commonly uses the statewide Alaska Albers NAD83 coordinate system, the local Division of Foresty commonly use the UTM coordinate system, the Fairbanks North Star Borough commonly uses the Alaska State Plane coordinate system. 26

27 A conic projection is used with two standard parallels touching the globe (scale factor of 1 along these parallels). 27

28 The statewide Alaska Albers projection has standard parallels at 55, 65 degrees north. Since the central meridian is -154 degrees, locations to the east of that are positive, to the west are negative. 28

29 Note the line where north-south is straight up and down is at the central meridian of -154 degrees. 29

30 Not bad for statewide applications. But we can do better for smaller areas. 30

31 The Mercator projection is good if you want a scale factor of 1.0 along a parallel such as the equator. The Transverse Mercator projection is good if you want a scale factor of 1.0 along a north-south meridian. 31

32 Alaska has ten UTM zones starting with zone 1 at -180 to -174, zone to -168, zone to - 126, and zone 60 from 174 to 180 east 32

33 UTM Zone 1 starts at -180 longitude, and is 6 degrees wide (-180 to -174 longitude) The scale factor at the central meridian is and it increases to km away from the central meridian. In Alaska, the worst scale factor is at the central meridian (0.9996) and it approaches 1.0 going east and west from the central meridian. A false easting value of 500,000 meters is always assigned at the central meridian. 33

34 The Alaska State Plane coordinate system has 4 degrees wide zones in contrast to the 6 degrees wide UTM zones. 34

35 Alaska State Plane zones 2 through 9 cover most of the state. 35

36 Zone 10 is custom made for the Aluetians/Alaska Peninsula 36

37 Zone 1 is custom made for southeast Alaska 37

38 The data frame coordinate system is set as soon as you add data to your data frame. The data frame coordinate system is the same as your first layer s coordinate system. Other layers are displayed correctly in that coordinate system, even if the source is a different coordinate system. 38

39 The data frame coordinate system is set as soon as you add data to your data frame. The data frame coordinate system is the same as your first layer s coordinate system. Other layers are displayed correctly in that coordinate system, even if the source is a different coordinate system. 39

40 If a coordinate system is unknown, ArcMap simply uses the X,Y coordinates and can not correctly display the same location from 2 coordinates sytems. 40

41 Define projection simply writes the projection to a *.prj file that is one of the many companion files that a shapefile is made of. Project and Batch Project change the X,Y coordinates by physically changing the projection. For example, Alaska Albers in meters to UTM Zone 6 in meters. 41

42 Remember, changing projections always involves and intermediate change back to geographic coordinates 42

43 Whenever a GCS datum changes with the Project tool, you need to specify the correct Geographic Transformation for Alaska. 43