AIM 8/15/19  7/16/20  procedures. The database is updated every 28 days. The data is available by subscription only and is distributed on CD − ROM or by ftp download. 3. digital − Visual Charts (d − VC). These digi- tal VFR charts are geo − referenced images of FAA Sectional Aeronautical, TAC, and Helicopter Route charts. Additional digital data may easily be overlaid on the raster image using commonly available Geographic Information System software. Data such as weather, temporary flight restrictions, obstacles, or other geospatial data can be combined with d − VC data to support a variety of needs. The file resolution is 300 dots per inch and the data is 8 − bit color. The data is provided as a GeoTIFF and distributed on DVD − R media and on the AIS website. The root mean square error of the transformation will not exceed two pixels. Digital − VC DVDs are updated every 28 days and are available by subscription only. FIG 9 − 1 − 14 Chart Supplement U.S. Geographic Areas 9 − 1 − 11 Types of Charts Available