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Latitude and longitude
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* July 16, 2009, 05:40:09 PM
I need to produce a drawing using latitude and longitude as the input coordinates.

The approximate center of the drawing will be S25.253041 E152.335761 and the extent is about 0.01 degrees radius

Is there a way of setting up TurbCAD so that I can directly enter coordinates, and secondly, to then get dimensions betweenpoints in meters?

I guess this is a surveying question.

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* July 16, 2009, 06:03:14 PM
#1
To change to surveyor go to options- angle- change the dropdown to surveyor. You may need to change the base angle to 90 if you want north up. To change the dimensions, go to the units/ tolerance page for the dimension tool and change the units there. Also you may want to make the tolerance to be at least .001 or more otherwise when you use the dimension tool the numbers round off and are not always what you enter.
Myron TC16Pro

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* July 16, 2009, 07:45:06 PM
#2
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?

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* July 16, 2009, 08:14:34 PM
#3
Interesting problem -- not trivial but not too difficult. Don't think I'll be able to answer it today :-(

Henry H

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* July 17, 2009, 08:12:25 AM
#4
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.  ;D


http://en.wikipedia.org/wiki/Geographic_coordinate_system




« Last Edit: July 17, 2009, 08:18:05 AM by WillR »

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WillR
SW:TC 21 Pro Platinum, FM15, 20 Pro Platinum 64 Bit, also V 19.1 64Bit & 32 bit 17.2 and 18.1; Furniture Maker 14 and 10. HW: Vista 64 Ultimate, AMD 1090 (6 core) 8GB memory Plus also an AMD  8 core systemFX


* July 17, 2009, 04:49:23 PM
#5
Thanks for all the info and pointers.

The area I'm concerned with is only 1000meters by 500meters (3000ft x 1500ft) so I'm not bothered about the earth curvature.

I'm familiar with the WGS84. When I refered to a "datum" I meant it in the sense of using one point as the "origin" for my drawing, then converting all the other lat and longs as X and Y offsets from that single point.

I have found a tool that will give distance and bearing from one point to another, so I'll probably locate all my points that way (again, using one point as my "origin")


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* July 17, 2009, 09:22:17 PM
#6
When I refered to a "datum" I meant it in the sense of using one point as the "origin" for my drawing, then converting all the other lat and longs as X and Y offsets from that single point.

I have found a tool that will give distance and bearing from one point to another, so I'll probably locate all my points that way (again, using one point as my "origin")

Sounds good.

Henry H

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* July 19, 2009, 05:57:48 PM
#7
With my "Import Points" tool just get all of the data into a text file. In that file change all 'N's to '+'s and all 'S's to '-'s and all 'W's to '-'s and 'E's to '+'s. Then when you do the import, using the tool, select the "projection" appropriate for your location.

The tool converts the XYs to meters by default but you will need to be mindfull of your drawing units and set them to meters before you import the data.

 


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