122 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
ment along a meridian. A plus sign in this column means that the point moved toward 
the south. The seventh column shows the east and west component of the motion. 
A plus sign in this column means that the point moved toward the east. The sixth and 
seventh columns were computed by converting the changes in latitude and longitude, 
respectively, into meters. 
By combining the values given in columns 6 and 7, the direction and amount of the 
displacement were obtained as shown in columns 8,9,and 10. In column 8 the direc- 
tion of displacement is given, reckoned as geodetic azimuths are usually reckoned, clock- 
wise around the whole circumference from south as zero. In this reckoning, west is 90°, 
north, 180°, and east, 270°. Column 9 gives the amount of displacement in meters 
and column 10 gives it in feet. Column 11 shows the approximate distance of the point 
from the fault of 1906, measured approximately at right angles to the fault. In this 
column E indicates that the point is to the east of the fault and W that it is to the west. 
For example: The fifth line of table 1 indicates that during the earthquake of 1906 the 
Farallon Light-house moved 0.83 meter north and 1.57 meters west, or, in other words, 
moved 1.78 meters (5.8 feet) in azimuth 118°, or 62° west of north, and that it is 37 
kilometers (23 miles) from the fault of 1906 and to the west of it. 
In the heading, the expression “ Before 1868” refers to years within the interval 
1851-1866. The expression “After 1868” refers to years within the interval 1874-1891, 
and ‘£1906-1907” refers to dates within the interval July, 1906—July, 1907. 
The latitudes and longitudes given in tables are all computed upon the U.S. Standard 
Datum and differ somewhat from those now in use on the charts and maps of this region. 
They are, however, the latitudes and longitudes to which all charts and maps should ulti- 
mately conform. 
Table 1 shows the displacements which occurred on April 18, 1906; table 2 shows the 
displacements which occurred in 1868, and table 3 shows the total, or combined displace- 
ments in both 1868 and 1906. 
For some cases, as, for example, Point Reyes Hill, the separate displacements were not 
directly determined by the triangulation but only the combined displacements. In such 
cases, if probable values could be derived for the separate displacements, indirectly, by 
inference from surrounding points, they were so derived and placed in the table. In each 
case, such inferred displacements are clearly distinguished in the table from others which 
were determined directly by measurement, by leaving the third and fifth columns blank 
and by having the values in the sixth to tenth columns enclosed in parentheses. 
All of the displacements given in tables 1-3 are computed upon the assumption that the 
two stations, Mount Diablo and Mocho, remained unmoved during the earthquake of 
April 18, 1906. The reasons why this assumption is believed to be true will be set forth 
fully in a later part of this report. 
In the tables the points are separated into seven groups for convenience of discussion. 
Each group of points is fixt by a portion of the triangulation which may conveniently be 
considered as a unit in discussing the magnitude of the possible errors of the triangulation. 
The discussion of the observed displacements and the degree of certainty in regard to them 
is given after the tables and deals with each group in succession. 
The apparent displacements, as shown in the above tables, are of course in part due to 
the unavoidable errors in the triangulation and in part are doubtless actual displacements 
of the points. The triangulation furnishes within itself the means of estimating its 
accuracy. If the observations were absolutely exact, the sum of the observed angles of 
each triangle would be exactly 180° plus the spherical excess of that triangle, and more- 
over the computation of the length of the triangle sides would show no discrepancies, 
starting from a given line and ending on a selected line, but proceeding thru the various 
alternative sets of triangles which it is possible to select connecting said lines. In any 
