Map reading is the determination of aircraft position by matching natural or built-up features with their corresponding symbols on a chart. It is one of the more basic aids to dead reckoning (DR) and certainly the earliest used form of aerospace navigation. The degree of success in map reading depends upon a navigatorâ€™s proficiency in chart interpretation, ability to estimate distance, and the availability of landmarks.
Checkpoints are landmarks or geographic coordinates used to fix the position of the aircraft. By comparing the aircraft position to that of the checkpoint, the navigator fixes the aircraftâ€™s location. Arrival over checkpoints at planned times is a confirmation of the wind predication and indicates reliability of the predicted track and groundspeed. If the aircraft passes near but not over a checkpoint, the anticipated track was not made good. If checkpoints are crossed but not at the predicted time, the anticipated ground speed (GS) was in error. Prudent navigators are quick to observe and evaluate the difference between an anticipated position and an actual position. They must make corrections to maintain their intended course as soon as possible because small errors can be cumulative and may eventually result in becoming lost.
Before fixing each position, navigators should look for several related details around each checkpoint to make sure it has been positively identified. For example, if the checkpoint is a small town, there may be a lake to the north, a road intersection to the south, and a bridge to the east.
Generally, it is better to select a feature on the chart and then seek it on the ground rather than to work from the ground to the chart. The chart does not show all the detail that is on the ground, and one could easily become confused. Checkpoints should be features, or groups of features, that stand out from the background and are easily identifiable. In open areas, any town or road intersection can be used; however, these same features in densely populated areas are difficult to distinguish. Figures 6-1 and 6-2 compare various chart and corresponding photo areas and list the features to look for when identifying landmarks as checkpoints.
Use a chart for map reading that provides sufficient natural and built-up features to accurately position the aircraft. The Operational Navigation Chart (ONC), with a scale of 1:1,000,000, has excellent cultural and relief portrayal. For increased detail, a Sectional Aeronautical, with a scale of 1:500,000, or suitable scaled USGS topographic charts with a scale of 1:10,000, may be used.