http://basemap.nationalmap.gov/ArcGIS/rest/services/USGSTopo/MapServer/tile/${z}/${y}/${x} -20037508.34278700 20037508.34278700 20037508.34278100 -20037508.34278100 15 1 top EPSG:3857 256 256 C:/gdalWMScache/USGSTopo 2