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• And one more reminder: latitude should typically be expressed in radians, not degrees before feeding it into the cosine function, i.e. latitude in degrees *
Message 1 of 4 , Oct 17, 2008
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And one more reminder: latitude should typically be expressed in radians,
not degrees before feeding it into the cosine function, i.e.

latitude in degrees * 3.1415927 / 180.0 = latitude in radians

_____

From: yws-maps@yahoogroups.com [mailto:yws-maps@yahoogroups.com] On Behalf
Of Alan Brown
Sent: Friday, October 17, 2008 2:20 PM
To: yws-maps@yahoogroups.com
Subject: RE: [yws-maps] Meter per pixel values?

Historical background - the concept of a meter was defined as 1 /10,000,000
of the distance from the equator to the north pole. Assuming the earth is a
perfect sphere (it isn't, but it's within a percent or so), the
circumference is 40,000,000 meters.

_____

From: yws-maps@yahoogroup <mailto:yws-maps%40yahoogroups.com> s.com
[mailto:yws-maps@yahoogroup <mailto:yws-maps%40yahoogroups.com> s.com] On
Behalf
Of Alan Brown
Sent: Friday, October 17, 2008 2:12 PM
To: yws-maps@yahoogroup <mailto:yws-maps%40yahoogroups.com> s.com
Subject: RE: [yws-maps] Meter per pixel values?

At the most zoomed in level, at the equator, the precision is 0.000010728836
degrees per pixel.

Since the world is 40,000,000 meters in circumference, and there are 360
degrees in a circle, you multiple by 40,000,000 and divide by 360 to get
1.19209288 meters per pixel.

For zoom level 1, the meters per pixel is cosine adjusted for latitude -
i.e. 1.192 x cos(latitude) = meters per pixel.

For each zoom level you go out, the meters per pixel doubles - 1.192 x
cos(latitude) * 2^(zoom level - 1)

There's a reverse zoom level we sometimes use - i.e. the whole world is a
single tile at zoom level 1, and the most zoomed in level for pre-render
maps is level 18 (instead of 1). The reason for this change is that we now
have higher resolution satellite images available, for new zoom level
18,19,20 . old zoom level = 19 - new zoom level.

I don't believe the new zoom level is exposed via the api, however.

The difference in the zoom levels is a weird historical artifact - some zoom
levels were defined at slightly different scales before we generated tiles
(i.e., not changing 2x for every zoom level you go out). It's been
mentioned that some believe the mapimage api should behave differently, but
there's been no action on this.

- Alan

_____

From: yws-maps@yahoogroup <mailto:yws-maps%40yahoogroups.com> s.com
[mailto:yws-maps@yahoogroup <mailto:yws-maps%40yahoogroups.com> s.com] On
Behalf
Of Guy Morton
Sent: Thursday, October 16, 2008 5:09 AM
To: yws-maps@yahoogroup <mailto:yws-maps%40yahoogroups.com> s.com
Subject: [yws-maps] Meter per pixel values?

Is there a reference somewhere that tells us the meter per pixel
values for each zoom level for the Flex/Flash tile API?

Yes, I know they vary by latitude, but if we could at least get the
reference values at the equator that would make life much easier.

And yes, I've googled for this but had no luck.

Why is it so hard to find this basic information?

Guy

PS We're moving to the Flex component/tiled model and away from the
REST map API, and note that the zoom levels are not the same...why is
that?

PPS I have tried calling getMetersToPixels(1,currentLatitude) and I
don't believe the answers I am getting - as I understand your zoom
levels, each level is double the size of the previous one. This is not
what I get from getMetersToPixels()...

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