Add example of mapping longitude/latitude to tile

README.md

@@ -52,6 +52,42 @@ The Sentinel-2, Sentinel-1, and Landsat images are GeoTIFFS so they contain geor
 Other data does not have georeference metadata, but data at each tile is aligned, so the georeference metadata from the above images is applicable to the other data as well with only a resolution shift.
 
 
+Mapping Longitude and Latitude to Tile
+--------------------------------------
+
+Here is an example of mapping longitude and latitude to a tile.
+
+First install packages:
+
+    pip install rasterio shapely utm
+
+Then launch Python shell:
+
+    from rasterio.crs import CRS
+    from rasterio.warp import transform_geom
+    import shapely
+    import utm
+    # Define source location.
+    src_crs = CRS.from_epsg(4326)
+    src_point = shapely.Point(-122.331711, 47.648450)
+    # Get UTM zone.
+    _, _, zone_suffix, _ = utm.from_latlon(src_point.y, src_point.x)
+    epsg_code = 32600 + zone_suffix
+    dst_crs = CRS.from_epsg(epsg_code)
+    # Transform to UTM CRS.
+    dst_point = transform_geom(src_crs, dst_crs, src_point)
+    dst_point = shapely.geometry.shape(dst_point)
+    # dst_point is in projection coordinates (meters).
+    # Now convert to pixel coordinates at 1.25 m/pixel.
+    col = int(dst_point.x/1.25)
+    row = int(dst_point.y/-1.25)
+    # Print the prefix for the image filenames.
+    print(f"{epsg_code}_{col//512}_{row//512}")
+    # Print the prefix for the tar filenames to know which one to download.
+    # These group together many 1.25 m/pixel 512x512 tiles into one tar file.
+    print(f"{epsg_code}_{col//512//32}_{row//512//32}")
+
+
 Sentinel-2
 ----------