Scale Bar Overlay Concern

StephenBaier

I don't know if anyone else has noticed, but the Scale Bar Overlay for the JMap seems incorrect / misleading. The Scale Bar shows a 0 to n distance, such as 0 to 1 km, based on the zoom of the map. The bar is always broken into 8 sections and should show 1/8th of the overall distance, but they instead appear to show one tenth of the overall distance.

I noticed the issue by drawing out a line that is exactly 200m long, traveling directly East to West. Placing the line next to the 1 km Scale Bar, it is exactly two hashes long. Either the implementation of the bar is incorrectly only showing 8 hashes, when it should be showing 10, or the Scale Bar is highly misleading or incorrect altogether.

Anyone else noticed this? Is this a known issue?

MarkBaird

The fix is needed on the scalebar overlay. In the existing method calcMapResolutionInMetres(), add the following 2 lines for map with linear units.

// FIX for Scalebar issue with linear Spatial References
// This is then multiplied by the cosine of latitude to get a more accurate number
// of metres per degree.
getExtentCentre(map);
_resolutionInMetres *= Math.cos(getExtentCentreLatitude(map.getSpatialReference()));


   /**
     * Convert the current map resolution to a resolution in metres per pixel.
     * This is used when calculating the scalebar sizes.
     *
     * @param map
     *            Convert the current resolution of this map.
     */
    private void calcMapResolutionInMetres(JMap map) {
      _resolutionInMetres = map.getResolution();

        if (_mapUnits.getUnitType() != UnitType.LINEAR) {
             // We have angular units and want to convert to metres. We do
             // this by taking our resolution per pixel in degrees and
             // multiplying it by the number of metres in a degree at the
             // equator (111200). This is then multiplied by the cosine of
             // our latitude to get a more accurate number of metres per
             // degree. We can then use this number to convert our degrees
             // per pixel to a metres per pixel value.
             getExtentCentre(map);
             _resolutionInMetres = Unit.convertUnits(_resolutionInMetres,
                    _mapUnits, Unit.create(AngularUnit.Code.DEGREE))
                    * 111200
                    * Math.cos(getExtentCentreLatitude(map
                            .getSpatialReference()));
         } else {
             // we have linear units so this is a simple conversion.
             LinearUnit linearUnit = (LinearUnit) _mapUnits;
             _resolutionInMetres = linearUnit
                    .convertToMeters(_resolutionInMetres);
            // FIX for Scalebar issue with linear Spatial References
            // This is then multiplied by the cosine of latitude to get a more accurate number
            // of metres per degree.
            getExtentCentre(map);
            _resolutionInMetres *= Math.cos(getExtentCentreLatitude(map.getSpatialReference()));
        }
    }


Let me know if this works better.

Mark