分辨率 比例尺计算式

unit_per_grid：地面实际分辨率

unit_per_grid = 512 * Scale / (96 * 4374754)

var inchesPerUnit = new Array(1, 12,63360.0,39.3701,39370.1,4374754);
function Unit(){}    // per pix
Unit.PIX = 0;        //1
Unit.FEET = 1;        //12
Unit.MILE = 2;        //63360.0
Unit.METER = 3;        //39.3701
Unit.LILOMETER = 4;    //39370.1
Unit.DEGREE = 5;    //4374754