[udig-dev] uDig Scale Bar - Bug 1993

[Emily Gouge <egouge@xxxxxxxxxxxxxxx>]

https://jira.codehaus.org/browse/UDIG-1993

I did not actually write the scalebar code however I did do some work on 
the styling of the scalebar at some point.

This bug is a problem with the scale bar (I am pretty sure the distance 
tool is correct) and results from the way uDig computes the map scale. 
The scalebar is based on the map scale; however because of the way uDig 
computes map scale the scale bar results incorrect.

For Lat/Long everything seems okay.  (If you zoom out to the world you 
don't get great results but I think that can be expected).  The problem 
is when you have a different projection.  See 
ScaleUtils.caculateScaleDenominator(...)

Currently uDig computes the scale by computing the x scale, y scale then 
combining the two to get the diagonal scale.

double refWidthMeters = fromCrsToMeter(bounds.getWidth(), crs);
double displayMeterDistancePixels = refWidthMeters * dpi / 2.54 * 100.0;
double widthScaleDenominator = displayMeterDistancePixels / width;
double refHeightMeters = ScaleUtils.fromCrsToMeter(bounds.getHeight(), crs);
double displayMeterHeightPixels = refHeightMeters * dpi / 2.54 * 100.0;
double heightScaleDenominator = displayMeterHeightPixels / height;
return Math.sqrt(heightScaleDenominator*heightScaleDenominator + 
widthScaleDenominator*widthScaleDenominator);

I think this is incorrect and the scale should be computed by combining 
the raw x & y values then computing the diagonal scale:

double diaWidthUnits = Math.sqrt(bounds.getWidth() * bounds.getWidth() + 
bounds.getHeight() * bounds.getHeight());
double diaWidthPx = Math.sqrt(width * width + height * height);
double d1 = fromCrsToMeter(diaWidthUnits , crs);
double meter = (d1 * dpi / 2.54 * 100.0) / diaWidthPx;


Changing the way the scale is computed fixes the problems with the 
scalebar being incorrect, at least for the test case I was looking at. 
It might also effect this bug:
https://jira.codehaus.org/browse/UDIG-1953

Thoughts on the way the scale is being computed?

Emily







On 23/07/2013 4:33 PM, Jody Garnett wrote:
> The uDig team is in the middle of transitioning to hosting at
> locationtech.org - and issues involved in the migration are taking
> our priority right now.
>
> As for (https://jira.codehaus.org/browse/UDIG-1993) the scale bar is
> the work of Emply, I will assign the bug to her but I am not sure of
> her availability.
>
> Do you have any indication what is correct? The distance tool or the
> scale bar?
>
>
>
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