actual measurement being about two miles north of the town. The base 
was 2400 feet long and was measured twice by means of the Invar Tape; 
the probable error being 1/50,000._ The longitude of one end of the 
base was assumed, and the latitude calculated from astronomical 
observations. This was subsequently corrected by the telegraphic 
longitude reduced to the South East Gate of the City. 
(B) On our arrival at Yen-an Fu, the surveyor found himself at 
fault, and it was considered advisable to measure a new topographical 
base. This base was 8 miles long and connected with the longitude by 
occultation. 
Datum Level. 
From the T’ai-yiian base-line, five elevated points were fixed by 
triangulation, their vertical angles were taken with the theodolite, and 
their altitudes determined trigonometrically. The altitude of the end of 
the base from which they were determined was assumed to be 2700 feet. 
As there is only a very gradual rise to this point from T’ai-yiian Fu (alt. 
2600 feet as found by the French Railway Survey) we think that the 
height assumed may be taken as correct to within 50 feet. 
Plane-table Survey. 
The plane-table work was executed by Hazrat Ali on the }-inch 
scale. The country traversed was, generally speaking, of the very 
hardest that could be found for the plane-table. With the exceptions of 
the mountain country extending 80 miles west of T’ai-yiian, the range 
immediately to the west of Ku-yiian Chou in Kansu, and the mountains 
in the vicinity of Lan-chou Fu, the whole terrain was covered with loess. 
Such a surface with its bare, rounded hill-tops—all usually of the same 
level and rarely varying more than 100 feet—affords no salient features 
suitable for triangulation; whilst the deep gullies cutting it in every 
direction make access difficult. In addition the valleys of even the 
larger water-courses offer but very little better fields of view, being never 
more than a mile, and but seldom more than a few hundred feet, wide. 
In every way, aloess country taxes the abilities and patience of a surveyor 
to the utmost, and I do not believe that too much praise can be given to 
Hazrat Ali for the consistent and conscientious accuracy of his work. 
Never was he seriously out but once. At Yii-lin Fu the work checked 
out to within 5' of Long. and 4! of Lat.; but our last three marches before 
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