[Proj] Re: Graduated equidistant projections for convenient
mikeo2106 at msn.com
Sun Aug 5 14:22:05 EDT 2007
In your postings, youve told why you consider accurate distances to be
important, and you said, You certainly cant get the best distance
measurements and the equidistant property simultaneously.
But, for conic, azimuthal and cylindrical maps, equal-area projections
typically have about twice as much percentage scale variation as do
equidistant projections or conformal projections.
So your statement could be answered,
but you certainly can get better
distance measurements with equidistant projections than with equal-area
You said that data maps should be equal-area, and that you want the most
accurate directly-measured distances. You want two goals that are mutually
In your most recent posting, you say:
Your post is quite long. I can't really get into a whack-a-mole game of
responding to each of your points only to have several more spring up in
Actually there were only a few points in my posting (some were
considerately accompanied by clarification-examples), and each point was a
direct answer to one of your main objections to what I had said. .
1. I answered you statement that equidistant projections dont give the best
distances, where you were referring to distances measured directly from the
map (No, they only give the best such distances when compared to those of
equal-area and conformal projections).
2. When you said that its very uncommon for a map projection to satisfy two
metric criteria simultaneously, I pointed out that the sinusoidal does,
because its equal-area, and it gives accurate and easy directly-measured
lat/long co-ordinates. Accurate directly-measured lat/long positions are a
metric criterion no less than is equal-area.
3. When you kept complaining that the data map projections that I was
suggesting dont give the best directly-measured distances, I pointed out
that it isnt reasonable to judge a data map by how good a navigational map
it is. I pointed out that its difficult to find a scenario in which someone
needs accurate directly-measured distances from a nature guidebook range-map
or a rainfall-distribution map in an atlas. No one wants or needs accurate
directly-measured distances from those special purpose maps. Their only
purpose, their whole point, is to show where their zones are.
But, aside from that, an equidistant conic does very well by directly
measured distances. Equidistant conic was one of my data map suggestions,
though it wasnt my first choice.
4. Lastly, I said that the sinusoidal would please all data-map users, by
easily, accurately and directly giving the two kinds of information
(position and area) that people actually need, want and use from a data map.
Those were my points. I said them because I wanted to, not to get a reply.
Nor do I care _why_ you dont reply to them. But it isnt because I threw
out a careless and seemingly unlimited chaff of points to beset you like
moles popping up in your yard everywhere. As I said, they were a few direct
answers to your main criticisms of what I had said.
I hadnt attacked you or lowered myself to off-the-subject criticisms such
as you resorted to.
Having said what wanted to say, it isnt important whether or not you reply
to it. But its dishonest to resort to the claim that you didnt answer
because my points were too many and too frivolous to bother with. To
resort to that dodge, are we displaying some authority-conceit? Much better
if you had just not replied.
I will just note that I don't seem to have the trouble you have in
determining geographic coordinates on a map as long as the map comes with a
Its reassuring to hear that you dont have any trouble determining accurate
geographical co-ordinates from an azimuthal equal area map of a continent,
with a widely-spaced graticule, when the projections center (in map
co-ordinates and lat/long co-ordinates) and its orientation are not
On a medium-scale map (a whole state, for instance) it's easy enough to
arrive a lat/long coordinate accurate to a few seconds' accuracy in a minute
Maps showing such a small region constitute a tiny fraction of the data maps
published in atlases and guidebooks sold to the general public. So your
success with state maps doesnt help a whole lot.
It just takes two measurements and a short calculation. That's FAR easier
than trying to correct for the projection's vagaries in assessing distances,
whether the map is conformal or not.
except that no one needs or uses accurate directly-measured distances
gotten from a nature guidebook range map or a rainfall distribution map in
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