code: public boolean open( String fileName, String locationPath, String mapsetName ) {
          cellFile = nullFile = null;
  
          filename = fileName;
          this.locationPath = locationPath;
          this.mapsetPath = locationPath + File.separator + mapsetName;
  
          if (hasChanged) {
              /* Test the type of the map. */
              if (rasterMapType == -9999) {
                  if (!getRasterMapTypes(filename, mapsetPath)) {
                      return false;
                  }
              }
  
              if (compressed == 1) {
                  /*
                   * Read the header of the map and the addresses of the compressed rows.
                   */
                  try {
                      addressesofrows = getRowAddressesFromHeader(readHeader(cellFile));
                  } catch (IOException e) {
                      return false;
                  }
              }
  
              /* Ok. Get ready to read data */
              moreData = true;
          }
  
          hasChanged(false);
  
          return true;
      }
docs: /*
  (non-Javadoc)
  
  @see jgrass.io.MapReader#open(java.lang.String, jgrass.map.Mapset) this opens checks for the
  file existence, sets the active and map regions, defines the maptype, opens the map and
  extracts the header to extract the rowaddresse and check if everything is alright