That allows me to draw a line from a given point at the appropriate angle.
But what I need to do is to enter a point 'a' at a specific lat and long (S25.251581 E152.330874), and another point 'b' at a specific lat and long (S25.254448 E152.339945), and then draw a line between 'a' and 'b'.
And then get the length of the line in meters.
Or do I have to choose one point to use as datum, and then for each other point calculate the northing and easting offsets (in meters) relative to my datum point. Then use those values to create my drawing?
What type of a line? Curved or straight. Different answers.
On a map that shows a relatively small area, the UTM system is usually used -- as it avoids this issue since the "projection" does the map flattening work.
Without knowing the intended use of the information I know that I would never dare answer it.
The reasoning is that you may actually need to know the circular (elliptical?) distance i.e the distance across the ground... Perhaps you need to know the length of the chord that is connecting two points on the globe.
BTW the word datum has a specific meaning in mapping terminology. Not sure you are using it correctly.
You might see a datum referred to as WGS84 (GPS based Datum) NAD, 83, NAD27 -- ones that I use a lot anyway -- and that you don't.
If you are getting data from someone else -- you must work in the same datum. Otherwise you are referring to different points on any
map. If you have control of the data -- that is much easier.
Sometimes people just want to do a "proximity test" -- often for web marketing -- that can stand a "pretty close" answer by using any calculation method.
Use this info at your own risk!http://www.movable-type.co.uk/scripts/latlong.html
Using the UTM in WGS84 is usually the best bet -- but the answers go "wonky" unless all points are on the same map -- which use the same 50,000 meter offsets/origins... So you may have to switch map sizes (grids)... to keep consistent origins.
And after writing this I see just how silly it is to try to explain this with out using too much mapping terminology. http://en.wikipedia.org/wiki/Geographic_coordinate_system
« Last Edit: July 17, 2009, 08:18:05 AM by WillR »
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