Geografic-Information-System/lib/gis/area.c

/**
 * \file area.c
 *
 * \brief GIS Library - Area calculation functions.
 *
 * (C) 2001-2008 by the GRASS Development Team
 *
 * This program is free software under the GNU General Public License
 * (>=v2). Read the file COPYING that comes with GRASS for details.
 *
 * \author GRASS GIS Development Team
 *
 * \date 1999-2008
 */

#include <grass/gis.h>


static struct Cell_head window;
static double square_meters;
static int projection;

static double units_to_meters_squared = 0.0;

/* these next are for lat-long only */
static int next_row;
static double north_value;
static double north;
static double (*darea0) (double);


/**
 * \brief Begin cell area calculations.
 *
 * This routine must be called once before any call to
 * <i>G_area_of_cell_at_row()</i>. It perform all inititalizations 
 * needed to do area calculations for grid cells, based on the current 
 * window "projection" field. It can be used in either planimetric
 * projections or the latitude-longitude projection.
 * <br>
 * If the return value is 1 or 0, all the grid cells in the map have 
 * the same area. Otherwise, the area of a grid cell varies with the row.
 *
 * \return 0 if the projection is not measurable (ie. imagery or xy)
 * \return 1 if the projection is planimetric (ie. UTM or SP)
 * \return 2 if the projection is non-planimetric (ie. latitude-longitude)
 */

int G_begin_cell_area_calculations(void)
{
    double a, e2;
    double factor;

    G_get_set_window(&window);
    switch (projection = window.proj) {
    case PROJECTION_LL:
	G_get_ellipsoid_parameters(&a, &e2);
	if (e2) {
	    G_begin_zone_area_on_ellipsoid(a, e2, window.ew_res / 360.0);
	    darea0 = G_darea0_on_ellipsoid;
	}
	else {
	    G_begin_zone_area_on_sphere(a, window.ew_res / 360.0);
	    darea0 = G_darea0_on_sphere;
	}
	next_row = 0;
	north_value = darea0(north = window.north);
	return 2;
    default:
	square_meters = window.ns_res * window.ew_res;
	factor = G_database_units_to_meters_factor();
	if (factor > 0.0)
	    square_meters *= (factor * factor);
	return (factor > 0.0);
    }
}


/**
 * \brief Cell area in specified <b>row</b>.
 *
 * This routine returns the area in square meters of a cell in the
 * specified <b>row</b>. This value is constant for planimetric grids 
 * and varies with the row if the projection is latitude-longitude.
 *
 * \param[in] row
 * \return double
 */

double G_area_of_cell_at_row(int row)
{
    register double south_value;
    register double cell_area;

    if (projection != PROJECTION_LL)
	return square_meters;

    if (row != next_row)
	north_value = darea0(north = window.north - row * window.ns_res);

    south_value = darea0(north -= window.ns_res);
    cell_area = north_value - south_value;

    next_row = row + 1;
    north_value = south_value;

    return cell_area;
}


/**
 * \brief Begin polygon area calculations.
 *
 * This initializes the polygon area calculation routines. It is used 
 * both for planimetric and latitude-longitude projections.
 *
 * \return 0 if the projection is not measurable (ie. imagery or xy)
 * \return 1 if the projection is planimetric (ie. UTM or SP)
 * \return 2 if the projection is non-planimetric (ie. latitude-longitude)
 */

int G_begin_polygon_area_calculations(void)
{
    double a, e2;
    double factor;

    if ((projection = G_projection()) == PROJECTION_LL) {
	G_get_ellipsoid_parameters(&a, &e2);
	G_begin_ellipsoid_polygon_area(a, e2);
	return 2;
    }
    factor = G_database_units_to_meters_factor();
    if (factor > 0.0) {
	units_to_meters_squared = factor * factor;
	return 1;
    }
    units_to_meters_squared = 1.0;
    return 0;
}


/**
 * \brief Area in square meters of polygon.
 *
 * Returns the area in square meters of the polygon described by the 
 * <b>n</b> pairs of <b>x,y</b> coordinate vertices. It is used both for 
 * planimetric and latitude-longitude projections.
 * <br>
 * Returns the area in coordinate units of the polygon described by the
 * <b>n</b> pairs of <b>x,y</b> coordinate vertices for planimetric grids.
 * If the units for <b>x,y</b> are meters, then the area is in square meters.
 * If the units are feet, then the area is in square feet, and so on.
 * <br>
 * <b>Note:</b> If the database is planimetric with the non-meter grid, 
 * this routine performs the required unit conversion to produce square 
 * meters.
 *
 * \param[in] x array of x coordinates
 * \param[in] y array of y coordinates
 * \param[in] n number of x,y coordinate pairs
 * \return area in coordinate units of the polygon
 */

double G_area_of_polygon(const double *x, const double *y, int n)
{
    double area;

    if (projection == PROJECTION_LL)
	area = G_ellipsoid_polygon_area(x, y, n);
    else
	area = G_planimetric_polygon_area(x, y, n) * units_to_meters_squared;

    return area;
}