[Proj] Extended range TM usage
Gerald I. Evenden
geraldi.evenden at gmail.com
Tue Aug 26 20:41:09 EDT 2008
On Tuesday 26 August 2008 8:35:43 am support.mn at elisanet.fi wrote:
> "Gerald I. Evenden" <geraldi.evenden at gmail.com> wrote:
> > My question is can anyone supply a rational reason for the practical use
> > of an elliptical TM projection with extended longitude range.
> How practical is that, I don't know, but there is the situation where an
> user is zooming out a view so much that a very large area is visible. There
> can rise a situation where those extreme areas should be shown somehow. The
> accuracy does not matter in that case, since it is only show in relation
> with that middle section which is the main focus.
> This same situation applies basicly to all projections. Sometimes users
> zoom out the views very much and the extreme areas should be shown somehow.
> Sometimes clipping them away, or projecting to some limiting line, might be
> as good alternative. But my opinion is that it is better to show something
> than nothing even if it is a bit unaccurate.
> If somebody wants to have more aggressive limiting, he might use the error
> mechanism, where the library warns him about the error getting too large
> and that particular solution might start to clip those areas away.
> Regards: Janne
Zooming out seems to be a common reason for extended TM range. In this
situation, the view is probably a monitor screen so there is no need for an
elliptical TM in the first place. In addition, if unlimited out-zoom is
employed you probably need to change projection anyway---a view from space
might suggest the the near side perspective plot (nsper) for example.
Extreme, but the point I want to emphasize is that one does not have to stick
to the same projection in the zoom process.
I would advise that the projection selected for display be determined by the
extent and scale of the data. Small scale maps would often suggest
pseudocylindricals, or many others commonly used for continental or global
extents. Only close ups or scales larger than 1:250,000 suggest TM. I would
not bother with elliptical projections for display computer display purposes.
The only time something like elliptical TM or any other elliptical projection
is demanded is when making maps on stable base material to be overlain on
similar class material or published where such standards are required.
When working for the USGS the preparation of a "stand alone" sheet such as AMS
1x2 sheet or 1:25,000 scale maps, the graphics were always prepares with an
elliptical projection in common use by Topo Div. for that type of map and
with precision commensurate with the printing process. If a map was a page
illustration in an article or report I do not recall anyone ever raising the
issue as to projection nor the use of an elliptical projection---any
reasonable projection that properly displays the data can be used. Note: the
USGS maps I am talking about were for geologic data but often had cultural
The thing I want to emphasize here is that computer display and certain types
of map publication are two entirely different subjects with different
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
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