136 DLT f.OOLINALE OL AGE OM OGY: 
results of the very best are only generalizations, taking account 
of the larger irregularities of surface, but paying no attention to 
the lesser ones, hence it is necessary that at the outstart a thor- 
ough understanding should be had as to the degree of accuracy 
desired, so that the traverses may be arranged to touch all points 
of elevation or depression that come within these limits. All 
points at which observations are made should of course be accu- 
rately located on the map. Much use should also be made of 
descriptions and sketches made on the spot. 
Where railroads, or other lines of levels, cross the area, their 
profiles should be taken as base lines, but too much reliance 
should not be placed on their accuracy. Some of them are excel- 
lent and their results may be taken without question, but others 
are very poor. An error of one foot to the mile is not at all 
uncommon in these surveys, and much larger ones are frequently 
met with. One which I have in mind, the official profile of an 
Illinois road, has an error of nearly one hundred feet in about 
fifteen miles. 
The results of all observations having been recorded in their 
proper places on the map, they should be translated into contour 
lines, which will at once indicate the variations in surface. See 
Plates I]. and III. I would strongly advise that the preliminary 
contour map be made éy ¢he observer, and for each small section 
separately while the details of surface are fresh in his mind. These 
maps can then be brought together and re-drawn by more expert 
hands. 
It was my fortune during the years 1892-3 to direct a sur- 
vey of the entire state of Illinois, along the lines indicated above. 
The time allowed was so short that the work had to be pushed 
winter and summer, without regard to weather. The task was 
rendered much easier than it otherwise would have been, by the 
large number of railroad lines which cross the state in every direc- 
tion, and by the kindly courtesy of their officials, through whom 
I was enabled to obtain profiles of the lines. These profiles, after 
they had been brought into accord, formed admirable base lines 
from which to carry on the other work. Under these conditions 
