# [Proj] Re: Distance measured in Mercator projection

strebe at aol.com strebe at aol.com
Thu Jul 10 15:28:22 EDT 2008

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Glad this works for you. Don't hesitate to ask for the more complicated formula if you need greater accuracy — the calculation you have will be off by a percent or two because it assumes a spherical earth.

Regards,
-- daan Strebe

-----Original Message-----
From: Shannon Scott <sscott at locationinc.com>
To: PROJ.4 and general Projections Discussions <proj at lists.maptools.org>
Sent: Thu, 10 Jul 2008 4:02 am
Subject: Re: [Proj] Re: Distance measured in Mercator projection

1m / cos (phi) was the formula I was looking for.

We have an application that maps data based on a search radius ( ie 5
miles around Chicago ).

The maps show the search radius. We had been using UTM for the maps, so
I was able to plot the search radius with this piece of code:

<snip>

// convert miles to meters

bufdist = (miles * 1609.344);

// create circle

for (double i=0;i<360;i+=5) {

deltax = (Math.cos(i) * bufdist);

deltay = (Math.sin(i) * bufdist);

pntx = (centerPntx + deltax);

pnty = (centerPnty + deltay);

// plot point

}

</snip>

The maps have been converted to work as a google map overlay, so we need
to use the Mercator projection.

I have modified the way the search radius is calculated:

<snip>

// convert miles to meters

bufdist = (miles * (Math.abs(1609.344 * (1/Math.cos(latrad))
)));

</snip>

and everything looks good.

Thanks again.

Shannon

Ed McNierney wrote:

> Shannon -

>

> First, let me apologize for trying to respond (twice) when I was too
> rushed to do so properly.

>

> Since you say you’re not very familiar with map projections, it might
> be helpful for you to describe a little bit more about what you’re
> trying to do. Daan’s and Chris’s comments are both correct but could
> be confusing to a novice; the scale factor at a single point is the
> same in all directions for a Mercator projection, but strictly
> speaking only at that point. If you have another point in a different
> place, the scale factor at that point will be different from the first
> unless they’re at exactly the same latitude. So if you’re measuring
> the distance between two points at a non-trivial distance apart, a
> 500-mile line will, for example, change length on a Mercator map
> depending on its angle with the Equator. What I was trying to say in
> my first, poorly-worded reply, is that the set of points that are all
> 500 miles from a center point on a Mercator map will NOT form a circle
> on that map.

>

> If your question really is, “How can I measure the great circle
> distance between two points given their Mercator coordinates” then
> that’s rather tricky. And it will depen
d on how far apart those two
> points are, and how accurate you need your answer to be.

>

> You will find incredibly talented professional expertise on this list,
> but it’s hard for people to answer the question you didn’t ask. If you
> can describe – in general terms, rather than specific ones – what
> you’re trying to do and what questions you’re trying to answer, that
> will make it easier. If there are constraints on your solution (e.g.
> “I have to use Mercator so please don’t suggest an alternative
> projection that would make the math easier”) let us know as well. Thanks.

>

> - Ed

>

> Ed McNierney

> Groton, MA 01450

> ed at mcnierney.com

> +1 (978) 761-0049

> ------------------------------------------------------------------------

>

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