Use traditional paper maps to find your location? Generally, if you are prepared according to the above points, there will be no too dangerous situation. But there is always an accident! What should you do if you accidentally clear all waypoints and tracks in a panic, or have to use another backup navigation device when the GPS is unfortunately damaged? At this time, all the waypoints recorded electronically are lost, and GPS can only tell you the longitude and latitude of the current position (just like the guy at the beginning). Don't panic. Take out the paper map (or those satellite maps with labels printed before our departure), and select a point with clear longitude and latitude. Of course, it is better to mark the POI on the printing paper - if this is not available, you can only find the nearest integer longitude and latitude intersection on the paper map.

　　In a word, you find a point on the graph with known coordinates. Then start the GPS at hand, manually input the coordinates of that point, and define a waypoint. Then start the navigation function of GPS, and the target is that point. At this time, the general GPS receiver will automatically calculate the straight-line distance and azimuth to that point. Note: Azimuth angle is generally the angle of clockwise rotation with true north as 0 degrees. For example, facing the north, the northwest direction is a number between 270-360 degrees (the fourth quadrant), while the northeast direction is a number between 0-90 degrees (the first quadrant). According to the scale of the map, calculate the length of the linear distance from here to that point on the map, which is generally a number in centimeters. Take the known point as the center of the circle with a compass, and draw a circle with a radius that is so long to calculate the distance. Then make a vertical line (parallel to the nearest known meridian) through this known point. In this way, our current position is on the circle. According to the theorem that the sum of the internal angles of a triangle is equal to 180 degrees, we only know that what we are going to calculate is a right triangle. According to the measured azimuth, we can get the included angle between the known point and the longitude in any case. The known linear distance is the hypotenuse of a triangle. According to the trigonometric function formula:

　　Bevel X sin (included angle)=vertical edge or bevel X cos (included angle)=horizontal edge

　　Calculate any vertical side of the triangle. Draw two right angled sides on the map according to the length with a ruler. The point where you intersect the circle is where you are now. As long as your satellite signal error is small and the map is serious, the calculation error is generally quite small. Of course, I hope you can have a calculator to calculate trigonometric functions, or you have a similar function calculation program installed in your mobile phone. So you can find your position on the map. A simpler way is to draw a straight line according to the included angle of the known point, and the point intersecting the circle is the current position. However, the error of such marking is greater than that of the previous calculation, because the angle measurement of the protractor is generally not very accurate.

　　Note that in this method, the larger the scale of the map, the closer the selected known points are, and the more accurate they are - because the earth is round.

　　Finally, my suggestion is to go to the no man's land to explore. It is better to prepare two GPS devices for the future.

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