Geodesy, Map Projections and
Coordinate Systems
• Geodesy - the shape of the earth and definition of earth datums
• Map Projection - the transformation of a curved earth to a flat map
• Coordinate systems - (x,y,z) coordinate systems for map data
Spatial Reference = Datum +Projection +Coordinate system
• For consistent analysis the spatial reference of data sets should be the same.
• ArcGIS does projection on the fly so can display data with different spatial references properly if they are properly specified.
• ArcGIS terminology
– Define projection. Specify the projection for some data without changing the data.
– Project. Change the data from one projection to another.
Types of Coordinate Systems
• (1) Global Cartesian coordinates (x,y,z) for the whole earth
• (2) Geographic coordinates (phai,lamda, z)
• (3) Projected coordinates (x, y, z) on a local area of the earth’s surface
• The z-coordinate in (1) and (3) is defined
geometrically; in (2) the z-coordinate is defined gravitationally
How to find Lat & Lon
Finding latitude is easy enough. The first thing you need to do is measure the angle between the horizon and the sun when the sun is at its highest point, which is right around noontime on your watch. A quick look at your trusty tables tells you which line of latitude the sun should be above on that particular day. For example, let's say it's noon on December 21, and the sun is directly overhead. Well, on that day the sun is above the Tropic of Capricorn, so your latitude would have to be 23.5 degrees S.
It's a good thing, if you're a navigator, that the Earth spins around at such an even pace. Every hour it moves 15 degrees. This means that if the sun is above the longitude of 0 degrees at noon, one hour later it will be above 15 degrees West. Let's say that the sun is directly overhead and your chronometer, which was set to noon when you were at 0 degrees, says it's 3 o'clock position. In those three hours, the sun moved 15 degrees 3 times, or 45 degrees. So you're at 45 degrees West. Of course, the fact that the sun was directly overhead (which very rarely happens) made it especially convenient for finding your longitude, but you could have found your longitude anyway, with the help of your tables (published such as Nautical Almanac).
• Geodesy - the shape of the earth and definition of earth datums
• Map Projection - the transformation of a curved earth to a flat map
• Coordinate systems - (x,y,z) coordinate systems for map data
• Map Projection - the transformation of a curved earth to a flat map
• Coordinate systems - (x,y,z) coordinate systems for map data
• For consistent analysis the spatial reference of data sets should be the same.
• ArcGIS does projection on the fly so can display data with different spatial references properly if they are properly specified.
• ArcGIS terminology
– Define projection. Specify the projection for some data without changing the data.
– Project. Change the data from one projection to another.
• (1) Global Cartesian coordinates (x,y,z) for the whole earth
• (2) Geographic coordinates (phai,lamda, z)
• (3) Projected coordinates (x, y, z) on a local area of the earth’s surface
• The z-coordinate in (1) and (3) is defined
geometrically; in (2) the z-coordinate is defined gravitationally
How to find Lat & Lon
Finding latitude is easy enough. The first thing you need to do is measure the angle between the horizon and the sun when the sun is at its highest point, which is right around noontime on your watch. A quick look at your trusty tables tells you which line of latitude the sun should be above on that particular day. For example, let's say it's noon on December 21, and the sun is directly overhead. Well, on that day the sun is above the Tropic of Capricorn, so your latitude would have to be 23.5 degrees S.
It's a good thing, if you're a navigator, that the Earth spins around at such an even pace. Every hour it moves 15 degrees. This means that if the sun is above the longitude of 0 degrees at noon, one hour later it will be above 15 degrees West. Let's say that the sun is directly overhead and your chronometer, which was set to noon when you were at 0 degrees, says it's 3 o'clock position. In those three hours, the sun moved 15 degrees 3 times, or 45 degrees. So you're at 45 degrees West. Of course, the fact that the sun was directly overhead (which very rarely happens) made it especially convenient for finding your longitude, but you could have found your longitude anyway, with the help of your tables (published such as Nautical Almanac).
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