[GRASSLIST:4178] Re: Map import cluelessness

[Ian Macmillan]

John,
Those are good points, and probably all valid (although I thought the east-west
scale was always constant along a parallel).  Most of my knowledge about
projections however comes from using GRASS (I think that is a good thing).  And
as far as I know, GRASS mercator locations measure things in meters.  It kind
of makes sense, but it makes things hard to measure given that it is really
difficult to figure out how many meters away you are from a central meridian
when dealing with very large distances (the only thing mercator projections are
good for).  Almost seems like the only location that should use meters is UTM,
everything else should use degrees.

-i

Quoting John <jwdougherty at mcihispeed.net>:

> On Wednesday 11 August 2004 12:46, you wrote:
> > Chris,
> > I think I understand your problem, it sounds like you are trying to
> > georectify your mercator map in degrees.  However, mercator locations use
> > measurement units of meters only.  Degrees are not a valid way of measuring
> > distances in mercator locations in GRASS (kind of makes sense).  So for
> > example a location on the earth with a mercator map would be something like
> > 546783, 3156734 as easting, northing, NOT 28.5E, 75.2N.  One thing you
> > could try if you only have degrees on your map is to use i.target, etc.
> > from your xy location straight to your lat long location (make sure that
> > the datum and ellipsoid match on your lat-long location and your scanned
> > map).  That should do the trick I think. Try to use at a minimum 6 points
> > in i.points, there will be a lot of distortion in the map, and the more
> > points you define, the more correct the distortion will be.
> >
> Ian,
>
> After reading your post, I find myself wondering about this.  Historically,
> the projection was developed in the 16th Century, and at that time, meters
> simply weren't defined.  Next, a Mercator projection doesn't have a constant
> scale either north-south or east west, so map units are problematic.  The
> projection is conformal (I believe that is the term) and provides a true
> course, rather than true distance, which would be an equal-distance
> projection if I remember my class work properly.  Anyway, I don't think the
> Mercator project could have any standard unit of distance.  The UTM
> projections would be a special case.
>
> Anyway, are you sure about that?
>
> John Dougherty
>
>


-----------------------------------------------------
Ian MacMillan
Geological Sciences-UCSB

"insert witticism here"