[MetaCRS] RE: Distance reductions for cadastral mapping

[STEPHEN STANTON]

Hi Norm (and Landon),

Thanks guys, that's enough to get me started I think. I imagine that  distances longer than 1km should be really unusual (more likely to be <100m), so using the scale factor at the midpoint will probably do the trick. I figure I can check things by converting positions into TM and use my existing method to see how the results compare.

Steve

--- On Fri, 7/1/11, Norm Olsen <norm.olsen at autodesk.com> wrote:

> From: Norm Olsen <norm.olsen at autodesk.com>
> Subject: RE: [MetaCRS] Distance reductions for cadastral mapping
> To: "STEPHEN STANTON" <sstanton at btinternet.com>, "metacrs at lists.osgeo.org" <metacrs at lists.osgeo.org>
> Date: Friday, 7 January, 2011, 20:35
> Hello Steve . . .
> 
> CS-MAP provides an easy way to obtain the "grid scale
> factor" which is a measure of the scale distortion added by
> the projection at a specific point within thew domain of the
> coordinate system.  VIZ: 
> 
> Grid Scale Factor -- CS_scale
> Given a coordinate system name and a location in the form
> of a geographic coordinate, CS_scale will return the grid
> scale factor of the coordinate system at the specified
> location. CS_scale returns a negative one in the event of an
> error condition. In such cases, the cause of the error can
> be determined by examining cs_Error which will contain the
> CS-MAP numeric error code of the condition which caused the
> error. CS_errmsg can be used to obtain a textual description
> of the error condition. An error caused by the location
> being outside of the domain of the coordinate system will be
> indicated by a cs_Error value of zero. In this case, no
> textual description will be available.
> 
> Note that the coordinate provided as the second argument
> must be a geographic coordinate, i.e. latitude and longitude
> in degrees referenced to the Greenwich prime meridian.
> Longitude is the first element in the array, latitude is the
> second. (The third element is not currently used for grid
> scale calculations, but may be in the future.) As always for
> internal geographic coordinates, use negative values for
> west longitude and south latitude.
> 
> double CS_scale (Const char *cs_nam,double ll [2]);
> 
> You can use CS_cnvrt to obtain the geographic coordinates,
> if necessary.  Use it to convert your cartesian
> coordinates to "LL".
> 
> Technically, there are two grid scale factors:
>     A grid scale factor along the meridian
> (h)
>     A grid scale factor along the parallel
> (k)
> 
> In the case of a Conformal Projection (like the Transverse
> Mercator), the two scale factors are equal.  The scale
> factors will not be equal in other types of
> projections.  The scale factor returned for
> non-conformal projections is usually the more interesting of
> the two.  For example, for projections of the equal
> area type, the scale along the parallel is returned and this
> value is the reciprocal of the scale along the meridian.
> 
> If the distances being adjusted are short, (i.e. less than
> a kilometer) you can probably use the scale at either
> end-point without a problem.  For longer distances, you
> may wish to consider calculating the scale at the mid-point
> of the line segment being adjusted.
> 
> Hope this helps.  Good luck on your project.
> 
> Norm Olsen
> 
> -----Original Message-----
> From: metacrs-bounces at lists.osgeo.org
> [mailto:metacrs-bounces at lists.osgeo.org]
> On Behalf Of STEPHEN STANTON
> Sent: Friday, January 07, 2011 7:18 AM
> To: metacrs at lists.osgeo.org
> Subject: [MetaCRS] Distance reductions for cadastral
> mapping
> 
> I'm looking to incorporate support for different map
> projections as part of a cadastral data entry program.
> 
> In particular, I need something that will let me convert
> ground distances into distances on the map projection (or
> should I say ellipsoid..., or datum..., or mean sea level)?
> The software I'm currently using only works for Transverse
> Mercator - I figure out a couple of approximate positions,
> then use those positions to calculate a line scale factor
> that I can then apply to my observed distance.
> 
> Of all the MetaCRS projects, it looks like CS-Map is the
> most comprehensive, but also the most intimidating! On
> scanning the user guide, I see scale factor mentioned quite
> a bit, but I didn't spot anything that relates specifically
> to distance reductions. Can anyone point me in the right
> direction?
> 
> Steve