Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . 722log sin 5o = (^y+rto) sin 5o = i3-2i = + i3-2i log i log cos 5o 5 = 3-83 = +3-83 Apparent declination = 45° 38 37-48at time of observationNov. 9, 1890. REDUCTION OF LATITUDE OBSERVATIONS. 7-\Z 325. Reduction of Latitude Observations—With all thispreliminary work done, the final reduction of latitude obser-vations is a comparatively simple matter. Grouping theobservations by pairs, the mean declination of each
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . 722log sin 5o = (^y+rto) sin 5o = i3-2i = + i3-2i log i log cos 5o 5 = 3-83 = +3-83 Apparent declination = 45° 38 37-48at time of observationNov. 9, 1890. REDUCTION OF LATITUDE OBSERVATIONS. 7-\Z 325. Reduction of Latitude Observations—With all thispreliminary work done, the final reduction of latitude obser-vations is a comparatively simple matter. Grouping theobservations by pairs, the mean declination of each pair isobtained, the corrections for difference of micrometer readingsand levels are applied, with a small correction for differentialrefraction, and the result is the desired latitude. Applying the foregoing corrections to formula (149), wehave the following working formula for reduction of latitudeobservations: 0 = i(<5 + <?) + {M - Mf- + ^[(« + «) - {s + s)] ^ 2 2 (169) EXAMPLE.—REDUCTION OF LATITUDE OBSERVATIONS. (Station : Rapid, South Dakota. November g, i8go. Half rev. micrometer = div. level — ). NovemDer 9. Weighted mean = 44 04 CHAPTER XXXV. LONGITUDE. 326. Determination of Longitude.—Determining the lon-gitude of a point on the surface of the earth consists in find-ing the angle between the two meridian planes passingthrough the station and a reference meridian. In the UnitedStates, Greenwich, England, is generally accepted as the zeroof longitude. Time and arc are interchangeable (Art. 304),differences in longitude may be expressed in time or 24 hours equals 360°, i hour equals 15°, i minute of timeequals 15 and i second equals i 5 of arc (Tables LVI to LIX).Therefore the angle between the two meridian planes abovedescribed is the same as the differences of the local times ofthe two stations. Accordingly, to determine the longitude ofa station is to determine the differences between the local timeat Greenwich and
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