[Geomoose-users] Scalebar & Measure

Fri Dec 17 10:04:16 EST 2010

All:

I just discovered this issue. After applying Brent's fix below the measure tool appears to return the correct values, but there are other remaining issues.

I am using the 900913 projection for all data.

1. The scale bar is off. 500 meters on the scale bar measures to 431 meters, or a factor of ~0.86. This is not a critical issue, but it would be "nice" if it measured the same.

2. The select feature, using By Point, is a very clever way to get all features within a certain radius of a point location. However this function will return a radius that is shorter by about the same amount as the scale bar. This is an important issue. I have attached 3 screenshots illustrating the problem. 

The GM shot shows what is supposed to be a 1-mile radius, plus the new measurement (post-fix) to the most distant feature of interest.

The GE-EC shot shows the measured distance using an all-Google application.

The ArcGIS shot shows the actual size of a circle with an (presumably ArcGIS is authoritative) 1-mile radius in the same location. These data are in NAD83 UTM 15N.

I measured the scale factors on a 200 meter scale bar (1:6771) for my state (Louisiana) and found them to be about 0.86 at the northern extreme and 0.89 at the southern extreme.

Is there any way I can implement some type of automatic correction to the select tool, and any other tool that may be affected by this behavior? Certainly I can inform the users that they need to add x % to their chosen buffer, but I am not prepared to trust the users to really do that.

Thanks!
/Johan.

> -----Original Message-----
> From: Brent Fraser [mailto:bfraser at geoanalytic.com]
> Sent: Wednesday, May 26, 2010 1:53 PM
> To: Len Kne
> Cc: 'geomoose-users at lists.sourceforge.net'
> Subject: Re: [Geomoose-users] Scalebar & Measure
> 
> Len(and others interested in measuring),
> 
>    Here's my solution to the measure problem:
> 
> In main.js somewhere around line 138 add the two lines below marked with a
> "//BWF":
> 
> 	Tools['measure'] = new
> OpenLayers.Control.GeoMooseMeasure(OpenLayers.Handler.MeasurePath,
> measureToolOptions);
> 	Tools['measure'].geodesic = true;  //BWF
> 
> 	Tools['measurearea'] = new
> OpenLayers.Control.GeoMooseMeasure(OpenLayers.Handler.MeasurePolygon,
> measureToolOptions);
> 	Tools['measurearea'].geodesic = true;  //BWF
> 
> Thoughts?
> 
> Brent
> 
> 
> Brent Fraser wrote:
> > Paul, Len,
> >
> >    Our scale problem is a (mostly) map projection issue.  All map
> projections
> > introduce distortions in trying to represent a round(ish) Earth on a
> flat plane.
> >   Mercator is no different: it is like wrapping an up-right cylinder
> around the
> > Earth, projecting light from the center onto the cylinder
> > (http://en.wikipedia.org/wiki/Mercator_projection).  As you can imagine
> the
> > distortions get "worse" the further from the equator.   The distortions
> can be
> > modeled as a Point Scale Factor.  At the equator the Point Scale Factor
> is 1.0,
> > at latitude 60 degrees North (or South), the scale factor is 0.5!
> >
> >    So what's the solution?  Here's mine:
> >
> >    1. Enhance Proj4 (and Proj4js) to return the Point Scale factor
> (given the
> > projection and a location)
> >    2. Enhance MapServer and Openlayers to use the Point Scale Factor in
> > calculating distances and scale.
> >
> > I'm trying to drum up interest, but so no takers....
> >    http://osgeo-org.1803224.n2.nabble.com/Point-Scale-factor-service-
> td4938743.html
> >
> > http://osgeo-org.1803224.n2.nabble.com/Re-3265-Scale-dependent-layer-
> switching-error-with-Mercator-td4966902.html
> >
> > Best Regards,
> > Brent Fraser
> >
> > Len Kne wrote:
> >> The scale bar issue with the web Mercator projection is a limitation of
> >> OpenLayers, the application GeoMOOSE 2.x uses to render the map frame.
> >>   There has been discussion on the OpenLayers site about providing a
> >> patch (this link offers a patch http://trac.openlayers.org/ticket/1890
> )
> >> in the next version 2.9.  I'm sure GM will upgrade to OL 2.9, but not
> >> sure there is any schedule in place to do this.
> >>
> >>
> >>
> >> The cursor issue is also kind of fun because it seems each browser
> >> handles the custom icons differently.  Here's the attribute in the
> >> mapbook which defines the query for the Identify service:
> >>
> >>
> >>
> >> cursor="url('cursors/identify2.cur') 9
> 9,url('cursors/identify2.cur'),auto"
> >>
> >>
> >>
> >> This makes three attempts to draw the cursor, the first offsets the
> >> mouse click 9 pixels from the top and the left.  Firefox can use this
> >> value and seems to line the mouse click up pretty good with the
> >> crosshair.  However, IE cannot handle the offset adjustment, so it
> reads
> >> the second cursor definition.  There are tools like AT File Mole which
> >> can be used to edit the hotspot of a cursor.  Attached is the latest
> >> version I have been using for Identify service, again this seems to
> line
> >> the crosshair up pretty well with the mouse click.  If you are using
> >> Safari or Chrome, these setting don't seem to work at all and I would
> >> recommend not using customer cursors with those browsers.
> >>
> >>
> >>
> >> Len
> >>
> >>
> >>
> >> *From:* Wirth, Paul [mailto:Paul.Wirth at kniferiver.com]
> >> *Sent:* Saturday, May 01, 2010 9:03 PM
> >> *To:* 'geomoose-users at lists.sourceforge.net'
> >> *Subject:* [Geomoose-users] Scalebar & Measure
> >>
> >>
> >>
> >> Two questions:
> >>
> >>
> >>
> >> 1.       I have a web Mercator application, and I noticed that the
> >> scalebar does not display the correct distance.  For instance, when the
> >> scalebar shows a distance of 20 miles, it measures only 13.8 miles.  I
> >> know the measure tools are working correctly, so I assume this is a
> >> scalebar issue.  Any thoughts on how to fix this?
> >>
> >> 2.       I think this question was asked before, but I can't seem to
> >> find it on the e-mail list.  With the measure tools, the cursor does
> not
> >> start measuring at the "crosshairs" when the user clicks, but rather in
> >> the NW quadrant of the crosshairs.  Is there a way to change it so the
> >> measure start point will be on the crosshairs?
> >>
> >>
> >> -----------------------------------------------------------------------
> -
> >>
> >> -----------------------------------------------------------------------
> -------
> >>
> >>
> >> -----------------------------------------------------------------------
> -
> >>
> >> _______________________________________________
> >> Geomoose-users mailing list
> >> Geomoose-users at lists.sourceforge.net
> >> https://lists.sourceforge.net/lists/listinfo/geomoose-users
> >
> >
> > ------------------------------------------------------------------------
> ------
> > _______________________________________________
> > Geomoose-users mailing list
> > Geomoose-users at lists.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/geomoose-users
> >
> 
> 
> --------------------------------------------------------------------------
> ----
> 
> _______________________________________________
> Geomoose-users mailing list
> Geomoose-users at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/geomoose-users
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ArcGIS_1mile.png
Type: image/png
Size: 952477 bytes
Desc: ArcGIS_1mile.png
Url : http://lists.osgeo.org/pipermail/geomoose-users/attachments/20101217/5636f376/ArcGIS_1mile.png
-------------- next part --------------
A non-text attachment was scrubbed...
Name: GEEC_1mile.png
Type: image/png
Size: 1389314 bytes
Desc: GEEC_1mile.png
Url : http://lists.osgeo.org/pipermail/geomoose-users/attachments/20101217/5636f376/GEEC_1mile.png
-------------- next part --------------
A non-text attachment was scrubbed...
Name: GM_1mile.png
Type: image/png
Size: 308473 bytes
Desc: GM_1mile.png
Url : http://lists.osgeo.org/pipermail/geomoose-users/attachments/20101217/5636f376/GM_1mile.png