[Proj] Info on proj4 formulas

[Clifford J Mugnier]

In Southern Algeria, it's tens of meters.  

Truncations were common in the 19th century, and were used to minimize the computational load to develop projection tables.  As welding technology advanced and stiff-leg platforms got further out offshore, the old projection tables produced highly distorted non-conformal errors.  Since the projection and the tables were legislated, that was the basis for the concession block coordinates.  Some areas like North Africa have experienced the same problems on land rather than offshore.

This affects warfighters and surveyors.  Not often, but it happens and not just in North Africa.  OGP uses the French Army Truncated Cubic Lambert specifically for this application.  Similar variations occur with the transverse Mercator.

Cliff Mugnier
LOUISIANA STATE UNIVERSITY
________________________________________
From: proj-bounces at lists.maptools.org <proj-bounces at lists.maptools.org> on behalf of Mikhail Tchernychev <misha at mail.geometrics.com>
Sent: Friday, May 9, 2014 1:28 PM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Info on proj4 formulas

Well, it seems to me first part of your letter contradicts (1) and (2).

Anyway, if there is negligible difference (say in order of cm) than what
could come wrong friendly fire or drilling?

But I have to admit that consequences could be severe for surveyor in
two cases:

1. there is a legal requirement to use specific formulas.  In this case
we are in trouble
even the formulas yield exactly the same results but not recommended
ones were used.

2. If required formulas were distorted on purpose.   Then this is just a
case of classified information.


Best Regards,
Mikhail

On 5/9/2014 10:59 AM, Clifford J Mugnier wrote:
> Variations in projection formulae can be an academic exercise or a catastrophe, depending on the application.  Most GIS applications can use practically any truncation of the infinite series (4th order, 5th order, 6th order, etc.) with little to no effect to day-to-day work.  There are two exceptions:
>
> 1.)   Military.  Always use whatever is currently promulgated by NGA for NATO/SEATO applications.  This helps avoid “friendly fire” incidents among units that provide indirect fire support.
>
> 2.)   Oil and Gas.  Exploration and Production concessions leased by national governments use map projections for Grid Systems as legal coordinate systems sometimes based on specific truncations.  When that is in effect, the use of other truncations (especially in offshore areas) can produce catastrophic errors in positioning.
>
> Sometimes, mathematical distortions at various distances from the projection origin are on purpose!
>
> Cliff Mugnier
> LOUISIANA STATE UNIVERSITY
> ________________________________________
> From: proj-bounces at lists.maptools.org <proj-bounces at lists.maptools.org> on behalf of Hermann Peifer <peifer at gmx.eu>
> Sent: Friday, May 9, 2014 12:47 PM
> To: PROJ.4 and general Projections Discussions
> Subject: Re: [Proj] Info on proj4 formulas
>
> On 2014-05-09 16:31, Nick Ves wrote:
>> Intresting,
>>
>> But now I'm wondering if the OGP's "Geomatics Guidance Note Number 7,
>> part 2 Coordinate Conversions and Transformations including Formulas"
>> [0] has been taken into account in the realizations of the formulas.
>>
>> [0] http://www.ogp.org.uk/pubs/373-07-2.pdf
>>
> At least Thomas Flemming mentions an earlier version of the guidance
> note, see http://trac.osgeo.org/proj/browser/trunk/proj/src/PJ_krovak.c
>
> Hermann
>
>
>
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