function NLWISMeasureWRegisterForEvent(){
    goCWCJSAPI.RegisterEvent(MAP_EXTENT_CHANGED, "NLWISMeasureWMapExtentsChanged");
}

// called to update the NLWISMeasure global vars
function NLWISMeasureWMapExtentsChanged(){
    NLWISClearPoints();

    gRulercMapWidth = goCWCJSAPI.oMap.width;
    gRulercMapHeight = goCWCJSAPI.oMap.height;
    gRulermapMinX = goCWCJSAPI.oMap.minx;
    gRulermapMaxY = goCWCJSAPI.oMap.maxx;

    dfPixelSize = (goCWCJSAPI.oMap.maxx - goCWCJSAPI.oMap.minx)/goCWCJSAPI.oMap.width;
    szMapUnit = goCWCJSAPI.oMap.GetUnitString(goCWCJSAPI.oMap.units);
    if (document.forms[0].NLWISMAPPROP_UNITS != null){
        gRulerUserUnit = document.forms[0].NLWISMAPPROP_UNITS.value;
        gRulersUnits = NLWISMeasureAbbrev(gRulerUserUnit);
    }
    gRulergblPixelSize = goCWCJSAPI.oMap.ConvertUnit(szMapUnit, gRulerUserUnit, dfPixelSize);
}