[OpenLayers-Users] OpenLayers.Control.ScaleLine accuracy issue

[Heidt, Christopher M.]

Have you tried using bounds functionms to get your corners?

Maybe:
var coordArray = map.getBounds().toArray();
Var ws = coordArray[0]+' '+coordArray[1];
Var en = coordArray[2]+' '+coordArray[3];

-----Original Message-----
From: users-bounces at openlayers.org [mailto:users-bounces at openlayers.org]
On Behalf Of plen
Sent: Tuesday, January 06, 2009 9:37 PM
To: users at openlayers.org
Subject: Re: [OpenLayers-Users] OpenLayers.Control.ScaleLine accuracy
issue


Christopher,

Thanks for the reply.  I see that you work for MetaCarta so you are not
as confused about planar vs geodesic vs degrees vs projected units etc
etc as I am.  A little background as to how we are using OpenLayers and
what led me to this question....

We are using a government mapping interface API (which is built upon
OpenLayers) for our 2D mapping solution. All base maps are populated by
them (Google Maps is not one of them).  Calls to do something with the
map are either through the government API or I can make some calls
directly to the OpenLayers API.  

I needed to find the map viewport's diagonal (SW to NE corner).  To do
this, I was comparing two different methods:  The OpenLayers distanceTo
function and another third party function that determines the length of
2 points on a sphere.  The distanceTo function always returned the same
distance value, regardless of where I was on the map while the other
method always returned a different value.  This goes to the flat map vs
a sphere thing.  The reason why I needed to viewport diagonal is that I
need to determine the SW & NE coordinates for the viewport at x zoom
levels out from the current zoom level.  For example, the viewport might
be at zoom level 13 and I need to find what the viewport's SW & NE
coordinates are if I were out at zoom level 10.  To do this I get the
bearings of the SW --> NE and NE <-- SW points and then determine the
distance out to the previous zoom level (for our 15 zoom levels I found
that the distance from the NE corner of zoom level x to the NE corner of
zoom level x-1 was half of the diagonal distance).  Using the bearing
and the distance out, I can caluculate wher the SW & NE coordinates are
for the zoom level I need.  In my tests, if I used the formula that
generated the diagonal based on 2 points on a sphere, the newly
generated SW & NE coordinates matched up exactly.  When I say exactly, I
meant that I could plot the new points on the map and then draw a
colored polygon based on the viewport coordinates for the zoom level I
was trying to go to and the points would be at the SW & NE corners.  If
I used the distanceTo function to determine the viewport diagonal, the
generated points would be further out and not match.  The distanceTo
function, like the map scale, seem to be based on a flat map.  

Now to your response..... while I am a little confused over what degrees
vs projected units is, I understood you to indicate that the OpenLayers
map is a 2d flat map and the scale is based on that premise.  My
confusion now is that it seem that several OpenLayers mapping functions
are based on degrees. 
I say that because, for example, get the area of a polygon (using the
OpenLayers API) returns a value in square degrees.  The distanceTo
function also returns a value in square degrees.  Also, because my newly
generated points that were based on a distance determine by 2 points on
a sphere (rather than a flat map) exactly matched the OpenLayers zoom
level corners makes me think that OpenLayers must deal with degrees on
the map.

That all being said, my head just spins when thinking of the whole issue
of geographic mathematics/algorithms.  Now, I don't really know what to
make of what I mentioned above.  I have to make the functionality for
our mapping requirements as accurate as possible and it doesn't seem
that I can use some functions that deal with map from a flat perspective
along side with other functions that deal with the map as a sphere.  I
am still trying to get my hands wrapped around this issue and my forum
question on the line scale was part of this process.  Out of this I am
not even sure if I am asking another question or just validation.  In
any case, I appreciate your initial response and your time in taking my
question.

Thanks - Peter Len


--
View this message in context:
http://n2.nabble.com/OpenLayers.Control.ScaleLine-accuracy-issue-tp21179
80p2120559.html
Sent from the OpenLayers Users mailing list archive at Nabble.com.

_______________________________________________
Users mailing list
Users at openlayers.org
http://openlayers.org/mailman/listinfo/users