AIR NAVIGATION MAP READING

[Music] this film is made from a light bomber now over the English Channel and approaching the south coast of England it represents the map Reading done after a daylight raid on deep and after the pilot has been forced by enemy fighters to take avoiding action for a considerable time throughout this period the Navigator was concentrating on Manning a gun and as a result is uncertain of his position to 50 Mi and only knows he will strike the coast Within 50 Mi of beach head the aircraft is headed on a course of 350° through and the camera is pointing about 40° to starboard through the bomb aer's window in the nose of the aircraft it is shooting obliquely and taking in a strip of country about a mile and a quarter wide the field of view is therefore the same as when the visibility is only about 1,000 yard here is the coast coming into view now the Navigator has never seen these Cliffs before otherwise he would recognize them immediately what does he notice a bend in the coastline with one arm running in a direction of approximately 03 ° through and the other arm 260° true the map shows that there are only three bends in the coastline within 50 Mi of beachy head which can possibly fit the description celi Bill beachy head and d NES which is it the experienced Navigator knows immediately by the shape of the Headland but apart from that within a mile radius of the Headlands the map shows a railway at celsi Bill and two at whereas beachy head has none no Railway was visible so it must be beachy head immediately on identifying his position the Navigator plots his required track from the coast as ordered in his briefing this was to proceed to South End aerodome avoiding this prohibited area in the t's estery he therefore Roots himself first to T's Haven and from there to south end arod at the same time he measures the true direction of this required track from beachi head using his table plotter he finds it is no one one de true he now computes the course to steer to make good the required track when the navigational computer is set for the true air speed the Met wind speeds and Direction the Navigator sees that if he St a course of 01 de through the drift will be 10° to starboard which will make the track 011 de true which is the required track the computer also shows that the expected ground speed will be 172 mph a navigator usually times the instant of crossing the coastline but this is impossible with an oblique field of view so we will time when the Headland is in the middle of the field of view now time by the aircraft clock 10 23 hours now the Navigator can time Mark the required track to south end at time distance intervals of 5 minutes for a ground speed of 172 mph setting the slide rule on the back of the navigational computer so that the 60 Minutes Mark is against the 172 m per hour mark he can read off the distance covered in any required time he reads off 2 minutes time distance 5.75 mil and steps off this distance from the scale on the map then at 2 minutes time distance from Beach head he makes the first time Mark for 1025 hours with the slide rule still set for 172 mph he reads off the time distance for 5 minutes 14.3 mil and time marks the remainder of the route for round 5 minute times this will make it easier to judge the expected time of arrival or the ETA at any distance between time marks he now fills in the etas at each time mark But as will be explained later he does not yet Put a Ring round them the map is oriented so that the track is in line with the heading of the aircraft along Town should appear on the right just before 1024 hours judging by the suburbs coming into view there undoubtedly is a large town on the right looking at the map what is the next feature to expect here is a triangle of Railways just short of the 1025 time Mark there is no other like it on the whole map sheet try to keep a mental picture of its shape the ability to do this marks the skillful map reader on the right sliding onto the clock is a railway line surely this leads to the triangle the railways it is half a minute before 10:25 and the triangle should be showing up and here is the South Fork the third side though not yet clear to the camera would be obvious to the Navigator it cuts across the picture just this side of the road lined with houses now you can see it clearly right across the middle of the screen what is the next feature to consider what about the road with the houses along it it looks an important map Reading feature but is it here it is on the map distinguishable only by the the railway alongside without the railway it might be any one of these in a large area of the map there are hundreds of Roads running in much the same direction had we looked first at the map the large number of roads in the area would have made us realize how valueless they are this case shows up the danger of working from ground two map returning to the map what is the next feature to expect a quarterway between time marks the track reaches an area scattered with Woods have they any value for position fixing no not in this case because there are so many that if his circle of uncertainty were large the Navigator would have to study the shape of several hundreds before he could fix his position by them on the other hand if there were only five Woods in the whole area of uncertainty all as large and distinctive as this one they might be valuable it is a good half mile each way with this piece at one corner try to keep the mental picture of the shape of this wood it may come into view around 1026 e note the time does the shape of this wood mean anything to you have you still got this picture in your mind the wood as shown on the map one disadvantage of a wood is that its ground shape May not agree with the map part of it may have been cut down since the map was drawn regard a wood has a longterm crop which may be harvested at any time Woods like any of these small ones now on the screen are only 2 or 300 yards across and too small to be shown on the half million map yet they look immense from low-flying aircraft and might snare an inexperienced Navigator into working wrongly from ground to to map what a wonderful Landmark he says and prompted pinpoints himself at any wood he can find on the map near where he wishes he was he would then cheerfully pack up his map Reading till an unexpected ocean makes him sure his map was wrong the map and time marks show that some low Hills with a spot height of 550 ft should be coming into view before 1028 the bomber is over low Hills now for low Hills are always hard to spot from the air and only have a map Reading value if there are few others in the circle of uncertainty these Hills would not show up at all in the picture but for the longer Shadows on the northwest slopes what is the next feature to expect here is a small Lake half a mile long according to the time mark it should appear a few seconds after 10:30 Lakes of the this size are not large enough for landmarks unless the position is already fairly sure it is only half a mile long and may have dried up or flooded it may have been camouflaged this is the lake coming into view on the far side of the wood in the the picture it looks like a light colored field actually it has been drained and only liquid mud remains this shows how care must be taken in using small lakes it was in the middle of the field of view at 10:30 the Navigator records the time of reaching the lake and Rings it a ring time is a fact an unring time only a prophecy shortly after the 10:30 time Mark the track crosses a railway here is the railway a cutting is visible in the top center of the field of view now the Navigator asks whereabouts is this along the line isn't that a tunnel on the left yes it makes a good pinpoint but only if the position is already known there are at least 15 tunnels on the map sheet all running in the same direction on the map a line between the tunnel and beachead shows that the track made good is 4° to Port of the required track the Navigator changes course 8° to starboard 4° to counter the drift and 4 degrees to make good the drift that has already occurred the map shows that for 3 minutes after the tunnel there is nothing but woods and Roads which are practically worthless the next important feature is a railway across the track it runs between Red Hill and Ashford is nearly dead straight for 44 miles and is one of the most prominent features on the whole map sheet the lines close to it on each side indicates that before the war it it was a civilian air route for airliners on their way to Cen according to the time mark it is due at about a minute before 10:35 for e for here is the area we've been flying over since the tunnel you can see that there are only woods and Roads and no distinctive features but it is now 10:33 and the important Railway line should soon be coming into view here is the straight Railway coming into view our track crosses the railway where another line curves away and then runs off northwards even if the position had been uncertain to 20 mes this would have pinpointed it exactly you will remember that the shape of the junction was like this here are all the Junctions on the map between Red Hill and Ashford they are like a row of signpost it is obvious that this is the only one that corresponds with the Junction now in view