src/core/qgsscalecalculator.cpp

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/***************************************************************************
                              qgsscalecalculator.h
                 Calculates scale based on map extent and units
                              -------------------
  begin                : May 18, 2004
  copyright            : (C) 2004 by Gary E.Sherman
  email                : sherman at mrcc.com
 ***************************************************************************/

/***************************************************************************
 *                                                                         *
 *   This program is free software; you can redistribute it and/or modify  *
 *   it under the terms of the GNU General Public License as published by  *
 *   the Free Software Foundation; either version 2 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 ***************************************************************************/

#include <cmath>
#include "qgslogger.h"
#include "qgsrectangle.h"
#include "qgsscalecalculator.h"

QgsScaleCalculator::QgsScaleCalculator( double dpi, QGis::UnitType mapUnits )
    : mDpi( dpi ), mMapUnits( mapUnits )
{}

QgsScaleCalculator::~QgsScaleCalculator()
{}

void QgsScaleCalculator::setDpi( double dpi )
{
  mDpi = dpi;
}
double QgsScaleCalculator::dpi()
{
  return mDpi;
}

void QgsScaleCalculator::setMapUnits( QGis::UnitType mapUnits )
{
  QgsDebugMsg( QString( "Map units set to %1" ).arg( QString::number( mapUnits ) ) );
  mMapUnits = mapUnits;
}

QGis::UnitType QgsScaleCalculator::mapUnits() const
{
  QgsDebugMsgLevel( QString( "Map units returned as %1" ).arg( QString::number( mMapUnits ) ), 4 );
  return mMapUnits;
}

double QgsScaleCalculator::calculate( const QgsRectangle &mapExtent, int canvasWidth )
{
  double conversionFactor = 0;
  double delta = 0;
  // calculation is based on the map units and extent, the dpi of the
  // users display, and the canvas width
  switch ( mMapUnits )
  {
    case QGis::Meters:
      // convert meters to inches
      conversionFactor = 39.3700787;
      delta = mapExtent.xMaximum() - mapExtent.xMinimum();
      break;
    case QGis::Feet:
      conversionFactor = 12.0;
      delta = mapExtent.xMaximum() - mapExtent.xMinimum();
      break;

    default:
    case QGis::Degrees:
      // degrees require conversion to meters first
      conversionFactor = 39.3700787;
      delta = calculateGeographicDistance( mapExtent );
      break;
  }
  if ( canvasWidth == 0 || mDpi == 0 )
  {
    QgsDebugMsg( "Can't calculate scale from the input values" );
    return 0;
  }
  double scale = ( delta * conversionFactor ) / (( double )canvasWidth / mDpi );
  QgsDebugMsg( QString( "scale = %1 conversionFactor = %2" ).arg( scale ).arg( conversionFactor ) );
  return scale;
}


double QgsScaleCalculator::calculateGeographicDistance( const QgsRectangle &mapExtent )
{
  // need to calculate the x distance in meters
  // We'll use the middle latitude for the calculation
  // Note this is an approximation (although very close) but calculating scale
  // for geographic data over large extents is quasi-meaningless

  // The distance between two points on a sphere can be estimated
  // using the Haversine formula. This gives the shortest distance
  // between two points on the sphere. However, what we're after is
  // the distance from the left of the given extent and the right of
  // it. This is not necessarily the shortest distance between two
  // points on a sphere.
  //
  // The code below uses the Haversine formula, but with some changes
  // to cope with the above problem, and also to deal with the extent
  // possibly extending beyond +/-180 degrees:
  //
  // - Use the Halversine formula to calculate the distance from -90 to
  //   +90 degrees at the mean latitude.
  // - Scale this distance by the number of degrees between
  //   mapExtent.xMinimum() and mapExtent.xMaximum();
  // - For a slight improvemnt, allow for the ellipsoid shape of earth.


  // For a longitude change of 180 degrees
  double lat = ( mapExtent.yMaximum() + mapExtent.yMinimum() ) * 0.5;
  const static double rads = ( 4.0 * atan( 1.0 ) ) / 180.0;
  double a = pow( cos( lat * rads ), 2 );
  double c = 2.0 * atan2( sqrt( a ), sqrt( 1.0 - a ) );
  const static double ra = 6378000; // [m]
  // The eccentricity. This comes from sqrt(1.0 - rb*rb/(ra*ra)) with rb set
  // to 6357000 m.
  const static double e = 0.0810820288;
  double radius = ra * ( 1.0 - e * e ) /
                  pow( 1.0 - e * e * sin( lat * rads ) * sin( lat * rads ), 1.5 );
  double meters = ( mapExtent.xMaximum() - mapExtent.xMinimum() ) / 180.0 * radius * c;

  QgsDebugMsg( "Distance across map extent (m): " + QString::number( meters ) );

  return meters;
}