# Re: [OpenMap Users] Measuring Distance on Map

From: Don Dietrick <dietrick_at_bbn.com>
Date: Fri, 1 Jun 2007 11:58:45 -0400

Hi Frank,

The map is just a representation of the points on the globe. It has
no effect on the actual distance between the points on the globe,
i.e, zooming in on a map obviously doesn't change the real distance
between New York and Los Angeles, right? The projection is an
attempt at modeling a spherical object on a 2D surface, and each type
of projection has to take away some part of reality in order to do
that. With Mercator, distances on the map don't translate to
distance on the ground (OK, perfectly North/South lines do, but
that's all). As another example, UTM and LCC projections let you
translate map distance to actual distance, but only for a limited
area (inaccuracies start to creep in), and the angles between points
aren't a direct translation to bearing/heading, nor can you assume
that a straight line is of constant bearing.

Hope this helps,

- Don

On Jun 1, 2007, at 11:04 AM, F.Bayliss wrote:

> Don,
>
> What you said make perfect sense. But, the map shown on the screen
> is flat and not wrapped around the globe. When I display the
> Graticule layer the distance (in pixels) between the Longitude
> lines are the same as you go north/south. So isn't the mileage the
> same also?.
>
> Thanks
> Frank
>
> Don Dietrick wrote:
>> Hi Frank,
>>
>> The distance measurements have nothing to do with the projection.
>> They are based, rightfully so, on elipsoid (Geo) and spherical
>> (GreatCircle) models. As you move north, distances between
>> longitudes get smaller (look at a globe with graticule lines and
>> measure the physical distance on the globe between longitude lines).
>>
>> Hope this helps,
>>
>> Don
>>
>> On May 31, 2007, at 9:55 AM, F.Bayliss wrote:
>>
>>> All,
>>>
>>> I'm confused. I'm using the Mercator projection and displaying
>>> the US in my application. I wanted to know the distance (miles
>>> and kilometers) between two points. I've tried using
>>> Geo.distanceNM & KM and I even copied some code from
>>> DistanceMouseMode using GreatCircle.spherical_distance. What I've
>>> found is the distance between two identical length longitudinal
>>> lines at different latitudes have different length. Why? Isn't
>>> the map displayed flat so the width, lets say - Wyoming being
>>> close to square, is the same width at the top and bottom borders?
>>> Help.
>>>
>>> Thanks
>>> Frank
>>>
>>> --
>>> [To unsubscribe to this list send an email to "majdart_at_bbn.com"
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>>
>>
>>
>> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>> Don Dietrick, dietrick_at_bbn.com
>> BBN Technologies, Cambridge, MA
>> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>>
>>
>
> --
> [To unsubscribe to this list send an email to "majdart_at_bbn.com"
> with the following text in the BODY of the message "unsubscribe
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Don Dietrick, dietrick_at_bbn.com
BBN Technologies, Cambridge, MA
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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Received on Fri Jun 01 2007 - 11:59:10 EDT

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