CHAPTER V. 
METHOD OF COMPILING THE MAP, WITH LIST OF PRINCIPAL DETERMI- 
NATIONS OF LONGITUDE. 
THE compilation of a map exhibiting the present state of our knowledge of the topography 
and hydrography of the territory of the United States, from the Mississippi river to the Pacific 
Ocean, is attended with two perplexing difficulties. First, the determination of what is relia- 
ble information; and second, the reconcilement of those discrepancies which are found to exist 
- even in maps of reliable explorers. Comparatively few points in this large area have their 
latitudes and longitudes determined with precision; and the surveys and explorations vary in 
accuracy, by almost insensible degrees, from the determinations of a boundary line to the crude 
information of the Indian, or the still more vague representations of the imaginative adventurer. 
In some large sections we possess no information, except from uncertain sources. In these 
parts the rule was adopted to leave the map blank, or to faintly indicate such information as is 
probably correct. 
Where discrepancies are found on comparing the maps of reliable explorations, especially in 
relation to geographical positions, the principle has been carried out of considering that ex- 
plorer’s map the most accurate whose experience and means of observation were the most 
perfect. Where these advantages appeared equal, a mean of the results was adopted. In 
other cases, a less proportionate value was given to the inferior, and in some cases it was even 
rejected. It is evident that the combination of the materials of different maps in one has 
necessarily required some distortion of the originals, — in all cases, much caution was observed 
to make this distortion as little as possible. 
The scale on which the general map was drawn and engraved is that of 1 to 3,000,000, or 
47.35 miles toaninch. This is too small to adequately represent the topography and character of 
the country, except in a very general way; and exacts either a sacrifice of many important details, 
or a deviation from the adopted scale. Many streams are laid down that would not, in their 
proper proportions, have a width greater than the 100,000th part of an inch. It is thought, 
however, that the map will answer the purposes for which it was intended, and its size (4 feet 
by 3 feet 10 inches) renders it more convenient for Foro than if it were drawn upon a larger 
scale. 
The projection of the meridians and parallels of latitude has been made from the tables pub- 
lished in the annual report of Professor A. D. Bache, Superintendent United States Coast 
Survey, for 1854, and is known as the “ Polyconic method.” This projection admits of a correct 
application of the scale of distance to all parts of the map, in directions east and west, and also 
along the middle meridian. But as we recede from the middle meridian, the length of miles 
on the scale are somewhat too small. This difference is greatest on the northeast and north- 
west corners of the map. Thus the length of two degrees on the 124th meridian is about three 
miles greater than on the 106th or middle meridian. This distortion is, however, so small, 
that distances are practically correct for all azimuths. 
