and the stadia interval is 6.23 feet. In the table under 3°TOPOGRAPHIC MAPPINGand opposite 26’, note that the multiplier for horizontaldistance is 99.64, while the one for difference inelevation is 5.98. If the final distance is ignored, thehorizontal distance isThe difference is elevation isTo these figures, add the corrections for focal distancegiven at the bottom of the page. For an instrument witha focal distance of 1 foot, add 1 foot to the horizontaldifference (making a total horizontal distance of 622feet) and 0.06 foot to the difference in elevation Thismakes the difference in elevation round off to 37.4 feet;and since the vertical angle has a negative (-) sign, thedifference in elevation is recorded as –37.4 feet.In the first column on the Remarks side of figure8-10, enter the elevation of each point, computed asfollows. For point 1, the elevation equals the elevationof instrument station D_{1 }(532.4 feet) minus thedifference in elevation (37.4 feet), or 495.0 feet.Subtract the difference in elevation, in this case, becausethe vertical angle you read for point 1 was negative. Fora positive vertical angle (as in the cases of points 12 and13 through 17 of your notes), add the difference inelevationThe remainder of the points in this example weredetailed in a similar reamer except for point 13. Whena detail point is at the same, or nearly the same, elevationas the instrument station, the elevation can bedetermined more readily by direct leveling. ‘That was thecase for point 13. As seen in the vertical-angle columnof the notes, the vertical angle was 0° at a rod readingof 5.6 feet. Therefore the elevation of point 13 is equalto the elevation of the instrument station (532.4 feet)plus the h.i. (4.8 feet) minus the rod reading (5.6 feet),or 531.6 feet.In the above example, as you recall, the transit wasinitially backsighted to point A and the zeros werematched This was because the azimuth of D_{1}A was notknown. However, if you knew the azimuth of D_{1}A, youcould indicate your directions in azimuths instead of inangles right from D_{1}A. Suppose, for example, that theazimuth of D_{1}A wasTrain the telescope on A andset the horizontal limb to readThen when youtrain on any detail point, read the azimuth of the linefrom D_{1 }to the detail point.Now you know how to perform and record atopographic survey, using the transit-tape ortranSit-stadia methods. Next, we will see how thedraftsman (who also might be you) prepares atopographic map. To enhance the explanation oftopographic mapping, we will also discuss someadditional field methods the surveyor uses.REPRESENTATION OF RELIEFOne of the purposes of a topographic map is todepict relief. In fact, this is the main feature that makesa topographic map different from other types of maps.Before you go any further, refresh your memory on thesubject of topographic relief. Relief is the term forvariance in the vertical configuration of the earth’ssurface. You have seen how relief can be shown in aplotted profile or cross section. These, however, areviews on a vertical plane, but a topographic map is aview on a horizontal plane. On a map of this type, reliefmay be indicated by the following methods.A relief model is a three-dimensional reliefpresentation-a molded or sculptured model, developedin suitable horizontal and vertical scales, of the hills andvalleys in the area.Shading is a pictorial method of showing relief bythe use of light and dark areas to suggest the shadowsthat would be created by parallel rays of light shiningacross the area at a given angle.Hachures area pictorial method similar to shadingexcept that the light-and-dark pattern is created by shorthachure lines, drawn parallel to the steepest slopes.Relative steepness or flatness is suggested by varyingthe lengths and weights of the lines.Contour lines are lines of equal elevation; that is,each contour line on a map is drawn through asuccession of points that are all at the same elevation. Acontour is the real-life equivalent; that is, a line of equalelevation on the earth’s surface.All of these methods of indicating relief areillustrated in figure 8-12. The contour-line method is theone most commonly used on topographic maps.CONTOUR LINESContour lines indicate a vertical distance above, orbelow, a datum plane. Contours begin at sea level,normally the zero contour, and each contour linerepresents an elevation above (or below) sea level. The8-12

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