96 METHOD OF COMPILING THE MAP. 
States Coast Survey, and is about 122? 47’. The meridian through Mount Pierce has not as 
yet determined any points of particular value. 
The United States Coast Surveys on the shores of the Pacific and Gulf of Mexico, are copied 
on the general map wherever they have been extended. The work, however, is just beginning— 
large portions of the coast being only fixed by preliminary reconnaissances—the outlines of 
the shore on their maps are therefore frequently changed as their surveys progress. 
By means of the United States Land Office surveys, and a few carefully determined astro- 
nomical positions, the longitude of most of the starting points of the different exploring expe- 
ditions on the east, have now been geographically fixed; and by means of the Land Office and 
Coast Surveys, the termini of many of them on the west. The maps of the surveys of the 
boundary between Mexico and the United States have been reduced to the general map as 
received from the office of those surveys; the work forming the southern border of the compi- 
lation, From this line I shall proceed north in describing the manner in which the. map has 
been filled in, and in so doing shall take up the different surveys without regard to the date of 
their prosecution, selecting the most reliable first. ln doing this the strict order of relative 
position must occasionally be abandoned, in order to settle the longitudes of some points com- 
mon to several explorations. | i 
Texas first claims attention. Colonel Johnston's published map, (Senate Executive Docu- 
ment, No. 64, 1st session 31st Congress,) as well as the manuscript maps of his subsequent 
explorations, are all made under the supposition that the longitude of San Antonio de Bexar is 
But his subsequent observations with a transit gave for this longitude 98° 25’, or 15’ 
east of that on the map. This would seem to require that the reconnaissances represented on 
that map should all be moved 15’ towards the east. The surveys of the Mexican boundary, 
however, show that El Paso and the mouth of Devil’s river were only about 10’ too far west 
on Colonel Johnston’s map. I have therefore moved the eastern part of Colonel Johnston’s 
map 15’ to the east, and the western portions but 10’, and reduced the intervening routes of 
reconnaissance accordingly.* The longitude of Preston then becomes 96° 38’. On Colonel 
Johnston’s map it is 96° 53’; on Captain Marcy's, 96° ۰, 
Captain Pope’s preliminary map, published with his Pacific Railroad Report, 8vo edition, 
differs very materially, in some of the positions, from those thus obtained from Colonel John- 
ston’s map, especially along the route of the survey of Lieutenant Michler, T. E.; but, after 
careful examination, I have adopted Colonel Johnston’s work. Captain Pope’s railroad recon- 
naissance route has been reduced conformably to the positions thus obtained. The portions of 
Texas south and west of Colonel Johnston’s map have been reduced from J, De Cordova’s map 
of Texas, dated 1849, which was the last edition available at the time of my compilation. 
The map of Lieutenant Parke’s exploration for a railroad route between El Paso and the 
Pimas villages, in 1855, + has been adopted and reduced without change. His survey was 
very carefully checked by a nearly continuous system of compass triangulations from peak to 
peak. His line was located by an ‘odometer and compass survey, corrected by astronomical 
observations, with a sextant for latitude. The geographical positions for El Paso and the 
Pimas villages were adopted as given by the United States Mexican Boundary Commission. 
9 The Mexican boundary maps retain the longitude of the lower crossing of the P. د ا‎ ee ا‎ 
[ Lieutenant Parke's surveys on this route, in 1854, as mapped did تیان‎ thi tius Docanidnt T9; intial a 
in our last reduction. Corrected, however, in his position of Dos Cabesas, and with the Rio Arivaypa taken from his last 
survey, it appeared on the first edition with the preliminary reconnaissances of the first Mexican Boundary Commission. 
The arrangements made in reducing these maps need not now be discussed, as they have been replaced. 
