Figure 7-1.—Diagram of a two-base altimeter survey.communication, a timepiece is needed for eachaltimeter. These timepieces are synchronized, and thealtimeter readings are taken at prearranged intervals.Figure 7-1 shows a diagram of the two-base methodwhen three altimeters are used. This figure shows theknown elevations of the lower (Sta. A) and upper (Sta.B) base stations. Altimeter readings at each of the basestations and at field station C are also shown. Thedifference in elevation is computed by direct proportion,using either the lower base or the upper base asreference. For example, to find the differences inelevation between Sta. A and Sta. C, we proceed asfollows:Then this result is added to the elevation of Sta. A, asshown in solution No. 1, figure 7-1. If we use the upperbase as a reference, you compute the difference inelevation by using the same method; but to computefrom Sta. B, subtract the result, as shown in solution No.2, figure 7-1.For a more accurate result, altimeter surveys shouldbe made on days when there is not much variation inbarometric pressure. Windy days when detached cloudsare traveling rapidly should be avoided becausealternating sunlight and shade over the survey area cancause fluctuations in the altimeter reading. Steadybarometric pressures generally occur on days withgentle winds and an overcast sky. The recommendedtime for observations is 2 to 4 hours after sunrise and 2to 4 hours before sunset. Midday observation must beavoided if possible. Remember, you must shade theinstrument at all times, and you must avoid jarring theinstrument suddenly during its transfer from one stationto another.TRIGONOMETRIC LEVELINGWhen you know the vertical angle and either thehorizontal or slope distance between two points, you canapply the fundamentals of trigonometry to calculate thedifference in elevation between the points. That is thebasic principle of trigonometric leveling. This methodof indirect leveling is particularly adaptable to rough,uneven terrain where direct leveling methods areimpracticable or too time consuming. As in any survey,the equipment that you will use in trigonometric levelingdepends on the precision required. For mosttrigonometric-leveling surveys of ordinary precision,angles are measured with a transit, or alidade, anddistances are measured either with a tape or by stadia,which you will study in chapter 8. On reconnaissancesurveys the vertical angles may be measured with aclinometer, and distances maybe obtained by pacing.The method used in trigonometric leveling isdescribed in the following paragraphs:7-2
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