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Aviation History
1963
1963 - 1857.PDF
m i» 345 HI* £T *>„ 4| 664 FLIGHT International. 17 October 1963 •Is o E ftOANft j AD ". &\, 1 ro «n» TK-rs* *» ate A ADAH* ite MM MM* Pi —-sffsrMTttA $ OftuMrtOP 4680 ilJpSU' CONTOUR ENVELOPES fig 7 A radio-navigation chart (part of, reproduced here approximately three- quarters the size of the coloured original). As in Fig 6, the few spots shown cannot possibly answer the user's essential question "How low is this or that area?" have not yet got a chart with the priorities right. The relegation of terrain to a separate chart (map) has not been decided on priorities, but is a historical accident arising from the way in which navigation was developed. The need at the present time is to re-think the priorities; re-build a "general navigation chart" (not calling it "radio-navigation" chart); and if it becomes congested choose for display elsewhere some of the less important items that do not have such serious consequences as do the sides of mountains. Some Possible Objections The above three examples of particular types of charts should be sufficient to show how the application of contour envelopes is intended; it seems unnecessary here to list every other type of aviation chart and to detail what should be done in every case—the important thing at this stage is to establish the principles. The most useful way to conclude, therefore, will be to give careful consideration to the objections that might be made to contour envelopes; they are roughly four:— (1) Contour Envelopes "a luxury" There must be many pilots and navigators who, having regularly used charts that show only spots, are inclined to take the view that there is nothing wrong in that system; they may therefore object to the contour envelope suggestion as being an unnecessary luxury. This view may even be put forward quite forcibly, but I believe it is precariously founded and its supporters will not have looked closely at what they are saying. Under the sub-heading "Spot Elevations Alone are Not Enough" I have tried to give the proof. The essential sentence is: "It is unsafe to fly past the side of a spot elevation at any lower height than one would fly over the top of it, and this can be true whatever one's distance off to the side." Now if a particular chart in fact shows spots in large numbers, there is a reasonable chance that the user will take in by eye not only the spots that lie obviously "on" his route, but also all those others that are relevant but which lie to the side; he will do this because commonsense shows it to be necessary. This procedure has worked more or less well (it can never be quite reliable) on the smaller-scale plotting-charts that often do have spots in considerable numbers. If, however, we try the same procedure on most other charts (on, say, a typical radio- navigation chart), the system utterly collapses: the number of spots is far too few; and their spacing on the paper is so wide that it is certain the user's eye will not take in those that lie off his route to the side; commonsense is unable to reach that far, because the distance-off that ought to be searched is so very much greater. You cannot champion the spot-elevation system and then be unconcerned about the density and distribution of the spots provided. (2) Congestion On this subject I have already said much in earlier paragraphs, especially with reference to the radio-navigation charts: but since this objection seems likely to be the most serious of the four, it is worth looking at again; moreover, the remarks which follow are not confined to one particular type of chart but have a general application to all. It is true that the addition of contour envelopes to charts as they are at present would in most cases be bound to increase congestion. But that is not a fair comparison. Since the present charts usually carry an inadequate selection of spots, the only fair comparison would be between (a) contour envelopes (plus those few spots coming under the contour rule given in an earlier paragraph) and (b) spots by themselves in adequate number to ensure that the user makes no false interpretation. The latter choice would mean, of course, a chart that is spotted all over. There is no doubt at all which system will win on congestion; indeed, in the case of the instrument approach chart it has already been demonstrated by the charts of the MoA (Fig 4). (3) Insufficient surveys Any specified requirement for contour envelopes of particular values can run into difficulties if it is fount. that some parts have not been sufficiently surveyed, but this is not i justifiable reason for taking no action; it has not, for instance, been used as a reason for providing no contours on the topographical maps. . If there is insufficient basic information for the drawing ot contour envelopes, then any corresponding portrayal by spot, alone is not only inadequate for the reasons already given, but i also likely to be concealing the basic deficiency: a chart can be pre duced showing only such spots as were available to the draughtsman <
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