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bobtait replied the topic: map to ground or ground to map
If you are using the principle of dead reckoning navigation it is normal to read from the map to the ground. The procedure is as follows:
1. Establish a DR position first by applying the best known ground speed to the time elapsed since the last positive fix. Assuming you have held a constant heading, this allows you to mark a point that represents where you SHOULD be according to simple mathematics.
2. Don't look outside yet! Check for features ON THE MAP in the vicinity of this DR position. Now you know what you EXPECT to see when you look outside at the ground. Remember that those features may be displaced slightly to the left or right, or ahead or behind you.
That's called reading from the map to the ground.
Reading from the ground to the map is fraught with problems. If you see a feature on the ground and then try to find it on the map, you are likely to find a number of possible candidates on the map that look something like the feature/s you have just seen. You should not use this technique unless you are unsure of your position.
You will find a full discussion on these techniques in the CPL Navigation book page 78 and RPL/PPL Volume 2 Page 264.
bobtait replied the topic: map to ground or ground to map
Yes, that's right. Let's assume you have established a DR position using time and distance since the last fix. Note that you can always establish a DR position, that's just based on simple arithmetic but, if the features on the map cannot be identified on the ground outside, the next step is:
1. Establish an "area of probability". Sketch lines about 15° or so either side of the track you set out to fly and about plus or minus 20% of the distance from the last fix to the DR position you have calculated. Providing you have held an accurate heading, there is a very high probability that you are somewhere in that area.
2. Now look outside and find some features that you can see near you and see if you can identify them on the map within the area of probability (read from the ground to the map).
You will find a full discussion on these techniques in the CPL Navigation book page 78 and RPL/PPL Volume 2 Page 264.
