Coordinates, Datums, and Map Projection

Transcription

1 Coordinates, Datums, and Map Projection

2 Two views on the World

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4 Intersect at 90 o angles

5 Latitude/ Longitude is a Spherical System

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7 Acceptable latitudes/longitudes i) W125 o ii) N92 o iii) lat -18 o 10 6 iv) lon -68 o v) N -15 o 19 10

8 Surface Distance

9 A problem You have historical field observations of land cover type across Minnesota, with lat/long. coordinates specified to the nearest 10 seconds of arc You have present-day satellite estimates of land cover, mapped in a 1km resolution raster Which of these data sets has the higher resolution (assume the resolution of the field observations equals the minimum difference discernible given the coordinate specification

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11 2011 Landcover

12 A Solution Which of these data sets has the higher resolution, 10 seconds of arc, or 1 km?

13 Can we use this formula to calculate the distance from Quito (0.181 S, W) to Libreville (0.4162N, 9.467E)? From Libreville to Ny-Alesund, Norway (78.923N, E)? From Ny-Alesund to Quito?

14 78.923N, E N, 9.467E 0.181S, W

15 Why are there 360 degrees in a circle (or around the globe? Why 60 minutes in a degree, and 60 seconds in a minute?

16 A chord equal to the radius spans 60 degrees of arc Six such chords complete the circle r r 60 o r

17 Latitude / longitude is NOT a Cartesian coordinate system Why?

18 Convergence

19 Coordinates are measured All measurements contain errors

20 Most accurate measure the latitude/longitudes today? 1 - Ground surveys 2 - Astronomical surveys 3 - Galactic radio waves 4 - GPS 5 - All of the above

21 Plates Are Moving, so Lat/ Lon of any point changes with time! GA Tech Dept of Geophysics

22 P Bird, 2009

23 Measurements have gotten so good, we can assimilate continental drift Current datums have an Epoch

24 Where is this?

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26 Our Earth isn t really shaped as a sphere (oid), or even an ellipse (oid)

27 Why? Law of Gravity: Force = g x Mass a x mass b r 2 Larger mass, stronger pull, and the Earth s mass isn t uniform throughout, e.g., near mountains, subsurface ore, deep mantle density variation 27

28 co2.cms.udel.edu geus.dk 28

29 In Addition: Isostatic Rebound 29

30 Survey Network, 1981 Geodetic Datum - an agreed upon reference surface AND a set of precisely determined locations ITRF Network

31 Lat/lon measurements made on surface, but reduced to the ellipsoid

32 Satellites/computer Revolution of the 1970s s Before: datums were national/continental, rarely recalculate origin at a geometric center of the Earth fixed a station or measurements, and propagated error from there Assumed a static network of points After: datums were generally global, recalculated often origin at the mass center distributed error across the network could incorporate/measure point velocities How Big is the pre/post satellite datum shift?!32

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35 Lat/lons change when we reduce to different ellipsoids

36 Why different modifiers and coordinate values? Different times, measurements, methods 36

37 Datums change through time - our estimate of the lat/ lon of objects improves, and the plates move

38 Datums Change through time - largest shifts from legacy, pre-computer, to modern datums in US, NAD27 to the NAD83(xx) or to WGS84(yy)

39 There are Three Main Families of Modern Horizontal Datums

40 Datum Conversions - HTDP

41 So, if you re not in the right datum, you may be off from a few inches to 6 feet (unless your in NAD27, then it may be 100 s of feet). We can fix this with a datum transformation, either......when we do a map projection...or for lat/lon using NGS program, HDTP, or NADCON

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44 How Big is the Shift from NAD83(86) to NAD83(96)? in Minnesota, about a foot

45 Data Mixing Question You have some old land-office survey latitudes/ longitudes you figure are accurate to the nearest 300 m, and are cast in NAD27 coordinates You want to combine it with current data in NAD83(2011) coordinates. Do you need to worry about the datum differences? What is the key question?

46 What is the datum shift in your locaction NAD27 to the NAD83(xx)?

47 Use the NAD27 to NAD83 shift map, or HTDP tool

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49 You have been provided raster land use data in NAD27 on a 1 km grid, and want to combine it with data in NAD83(86). Do you need to worry about datum differences? Your GPS is giving you readings in the latest WGS 84 coordinates, accurate to the nearest 10 cm. The rest of your data are in NAD83(96) coordinates. Do you need to transform datums?

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51 You have data in NAD83(HARN) coordinates, accurate to the nearest 10 cm. The rest of your data are in NAD83(2007) coordinates. Do you need to transform datums? You have data in ITRF88 coordinates, accurate to the nearest 50 cm. The rest of your data are in ITRF00 coordinates. Do you need to transform datums?

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53 How do we transform in Arc?

54 How do we transform in Arc?

55 With the projection tool, note, sometimes multiple datum shifts needed

56 Another example, multiple datum shifts

57 ArcMap, beware naming conventions, file location Text Should specify NAD83(1986), but most folks don t

58 For Modern NAD83, make sure you have the year modifier - if you dig in the Arc documentation, you can find the date and area

59 Three Surfaces: Ground, Geoid, Ellipsoid Remember, while lat/lon are on the ellipsoid heights measured relative to the geoid