[Fwd: [Mapserver-users] <PARAM NAME="PIXEL_SIZE" VALUE="???">]

[Daniel Morissette]

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Message-ID: <000e01c2b98a$68883ea0$3560063d at clickntrac>
From: "Steve McCrone" <mccrone at pc.jaring.my>
To: "Daniel Morissette" <morissette at dmsolutions.ca>
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Subject: Re: [Mapserver-users] <PARAM NAME="PIXEL_SIZE" VALUE="???">
Date: Sat, 11 Jan 2003 23:59:15 +0800
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Hey, thanks for your insight - the estimated value for my distance is
adequate. I made the calculation and used
the SetPixelSize() function - its working fine and changes the value
correctly when I zoom in/out.

I may be pushing my luck here but how do I access the distance measured once
the user finishes
with the tool (i.e. esc or double clicks). The text in the status bar is
cleared once this happens -
I wanted to use this measured distance in a java script function., and I
don't see a way of capturing
the status bar text as the mouse moves.

Steve
----- Original Message -----
From: "Daniel Morissette" <morissette at dmsolutions.ca>
To: "Steve McCrone" <mccrone at pc.jaring.my>
Cc: "mapserver-users" <mapserver-users at lists.gis.umn.edu>
Sent: Saturday, January 11, 2003 4:06 AM
Subject: Re: [Mapserver-users] <PARAM NAME="PIXEL_SIZE" VALUE="???">


> > Steve McCrone wrote:
> >
> > The pixel_size = ([maxx] - [minx])/[width] will give me how many
> > degrees (decimal degrees) is each pixel, But some further math has to
> > be done to somehow equate this to a distance. Now, I could "inverse"
> > between the two extents to get a metres value but I would essentiall
> > ybe bypassing the "built-in" measurement
> > <PARAM NAME="DIST_MSG" VALUE="Dist: %s m">.
> >
> > Still puzzled. I may have to resort to PHP mapscript to get further
> > control over my app.
> >
>
> Ah, your need to convert the pixel size from degrees to meters, and
> doing this accurately requires that you use the PROJ library because one
> degree of longitude is not the same lenght depending on how far you are
> from the equator.  So to get an accurate pixel size value you would have
> to use MapScript (Perl, PHP, etc.) or the php_proj module that comes in
> the php_mapscript directory to calculate the size of one pixel at the
> center of your map image.
>
> Now, if you can live with a very rough approximation you can go with the
> assumption that 1 degree is approx. 69 miles, or 110 km at the equator.
> So the formula becomes:
>
> pixel_size_in_meters = 110000*([maxx] - [minx])/[width]
>
> Daniel
> --
> ------------------------------------------------------------
>  Daniel Morissette               morissette at dmsolutions.ca
>  DM Solutions Group              http://www.dmsolutions.ca/
> ------------------------------------------------------------
>



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