# [OSRS-PROJ] Re: projection formula?

Strebe at aol.com Strebe at aol.com
Wed Oct 22 13:59:04 EDT 2003

```Chris Jessee <jessee at virginia.edu> writes:

>User mouse movement gives realtime lat lon readout.
>A measure tool provides distance and angle measure between two points.

I'm curious what you want the "angle" for. If you intend to measure direction
with it then you will fail. There is no projection on which you can measure
correct directions between any two points. If it is direction you want, then
you need to calculate the azimuth from the first point to the second. Gerald
Evenden mentioned Snyder's "Map Projections - A Working Manual". That reference
includes azimuth calculation formulae.

>The trouble begins when we try to use a base map in a
>Lambert_Conformal_Conic projection. The specifics of the projection are
>at the end of this email. To implement the functionality noted above I
>have 2 choices: re-project the map into a Geographic Coordinate system
>or dynamically calculate the difference between rectilinear screen
>space and the conic projection. On the first option I'm also

What you really want is the inverse projection. Inverse projections compute
latitude and longitude given the cartesian coordinates x and y. The same Snyder
reference provides inverse formulae for the Lambert conformal.

Since Snyder's volume may be hard to find in a hurry, you may also look at:

<A HREF="http://mathworld.wolfram.com/LambertConformalConicProjection.html">http://mathworld.wolfram.com/LambertConformalConicProjection.html</A>
http://www.codeguru.com/algorithms/GeoCalc.html

This all seems very elaborate for your project! It looks very good, though.

Regards,

daan Strebe
Geocart author
<A HREF="http://www.mapthematics.com">http://www.mapthematics.com</A>

Original message:

> Hi,
>
> I'm involved in a project that aims to offer an online tool with basic
> GIS functionality as follows:
>
> User mouse movement gives realtime lat lon readout.
> A measure tool provides distance and angle measure between two points.
> Point coverage can be loaded from external DB and plotted to a graphic
> base map.
>
> The tool will be built in Macromedia Flash. Implementing these features
> on a geographic coordinate system is relatively easy as this demo shows.
> http://jefferson.village.virginia.edu/salem/maps/
> click the map on the left.
> Flash player plug-in required.
> We ruled out many off the shelf products due to poor visual quality.
>
> The trouble begins when we try to use a base map in a
> Lambert_Conformal_Conic projection. The specifics of the projection are
> at the end of this email. To implement the functionality noted above I
> have 2 choices: re-project the map into a Geographic Coordinate system
> or dynamically calculate the difference between rectilinear screen
> space and the conic projection. On the first option I'm also
> considering a Platte Carre or Equidistant Cylindrical projection as
> this should be easier to plot lat lon. Any thoughts on what a
> appropriate Geographic Coordinate system might be to minimize visual
> difference with Lambert_Conformal_Conic? On the second option, can
> anyone point to a resource for formulas to convert
> Lambert_Conformal_Conic to Platte Carre or GCS_North_American_1983?
> I'll need the formula's since it all has to be implemented inside Flash
> to get the user realtime feedback.
>
>
> Coordinate system specifics
>
> Lambert_Conformal_Conic
> False_Easting: 200000.000000
> False_Northing: 750000.000000
> Central_Meridian: -71.500000
> Standard_Parallel_1: 41.716667
> Standard_Parallel_2: 42.683333
> Latitude_Of_Origin: 41.000000
>
> GCS_North_American_1983
> Datum: D_North_American_1983
> Prime Meridian: 0
> Datum: D_North_American_1983
> spheriod: grs 1980
> semimajor axis 6378137
> semiminor axis 6356752.3141403561
> angular unit: degree
> radians per unit: 0.017453292519943295
> prime meridian: Greenwich
>
>
> Thank you,
>
> Chris Jessee
> jessee at virginia.edu
>
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