Re: [OpenMap Users] What does the scale *really* represent?

On Tuesday, February 11, 2003, at 05:09 PM, Matt McKeon wrote:

> Don Dietrick wrote:
>> Hi Matt,
>> On Monday, February 10, 2003, at 12:08 PM, Matt McKeon wrote:
>> The scale is based on the
>> com.bbn.openmap.proj.Planet.defaultPixelsPerMeter (= 3272) setting.
>>
>> Each OpenMap projection uses this constant as a basis of creating
>> pixel spacings relating to scale, usually in the computeParameters()
>> method of a Projection.
>> Hope this helps,
>
>
> Hm. Kinda.
>
> I've investigated the usage of the scale property a little more.
> It appears to be the ratio of real-world pixel units
> (derived from pixelsPerMeter) to screen pixel units.
>
> It also looks like you can manually set the pixelsPerMeter
> on a projection, thus causing the scale property to be
> interpreted slightly differently. Unfortunately, this is an int;
> not something that can really be used directly when converting
> from a projection datum that specifies scale in terms of MPP.

The PPM was just a baseline for the scale as it was represented on the
screen.

> However, I guess that means the following formula would work.
> To convert a meters-per-pixel scale from an OGC projection
> ogcProj to an OpenMap scale in an OpenMap projection omProj:
>
> proj.getScale() == ogc.getMetersPerPixel() * proj.getPPM();
>
> Does this look right?

I think the gist is correct, and I wouldn't be surprised if it does
work.

- Don

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Don Dietrick, BBN Technologies, dietrick@bbn.com
10 Moulton Street, Cambridge, MA 02138
617-873-3031 [fax]-2794
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