We show how a system for video-rate parallel camera tracking and 3D map-building can be readily extended to allow one or more cameras to work in several maps, separately or simultaneously. The ability to handle several thousand features per map at video-rate, and for the cameras to switch automatically between maps, allows spatially localized AR workcells to be constructed and used with very little intervention from the user of a wearable vision system. The user can explore an environment in a natural way, acquiring local maps in real-time. When revisiting those areas the camera will select the correct local map from store and continue tracking and structural acquisition, while the user views relevant AR constructs registered to that map.

The inherent precision of spirit leveling has preserved its utility as a geodetic measurement system for over a century. While various instrumental and procedural modifications designed to enhance this precision have been introduced over the years, the basic measurement system has remained virtually unchanged since the mid‐nineteenth century. Possible systematic error has dictated the majority of the procedural and instrumental requirements associated with geodetic leveling; the physical source(s) of several of these errors remain poorly understood. Statistically independent random errors, which accumulate according to the square root of the survey distance, are generally controlled through redundancy and procedural randomization; they range from 0.5 mm L1/2 for the highest‐order modern leveling to about 6 mm L1/2 for the lowest‐order nineteenth‐century geodetic surveys, where L is the survey distance in kilometers. Height differences are conceptually distinct from observed or measured elevation differences in the sense that the former are uniquely defined, whereas the latter are path dependent, a distinction that arises from the nonparallelism of the equipotential surfaces of the earth’s gravity field. The number of possible height systems is virtually limitless. They include the systems of geopotential numbers and dynamic heights; although neither of these systems is geometrically informative, each provides perfectly valid height characterizations that may be especially useful in the solution of certain physical problems. The most generally used system of heights is the orthometric height system; the resulting heights are true geometric heights above the geoid. Normal height systems are referred to the quasi‐geoid rather than the geoid. Each of the various height systems meets the requirement of uniqueness, and none can be viewed as being conceptually superior. Conversion of the observed elevation differences obtained from leveling into uniquely defined height differences requires the application of a gravity‐dependent correction. Because gravity coverage in North America was generally sparse until recently, an approximation for this correction, which provides for the effects of the poleward covergence of the equipotential surfaces, has usually been used on this continent. Heights have been traditionally referred to mean sea level as a datum, a usage that implies coincidence between mean sea level and the geoid (or quasi‐geoid). Because the determination of mean sea level is dependent on the length of the observation period, because its definition makes no allowance for vertical crustal displacements or changes in eustatic sea level, and because its definition disregards the demonstrable existence of sea surface topography, local mean sea level generally departs from the geoid. This introduces errors in computed heights that probably equal or exceed those due to leveling. Repeated levelings continue to provide the best basis for determining terrestrial vertical displacements. These displacements are necessarily...

No abstract

No abstract

No abstract

Preliminary examination of the historic geodetic record has disclosed crustal uplift of 0.15 to 0.25 meter that apparently began around 1960 and has since grown to include at least 12,000 square kilometers of southern California. This uplift extends at least 150 kilometers west-northwestward along the San Andreas Fault from Cajon to Maricopa, southward from the San Andreas into the northern Transverse Ranges, and eastward from Lebec into and including much of western Mojave block. It seems to have grown spasmodically eastward from a center near the junction of the San Andreas and Garlock faults and has occurred largely within an area that has remained virtually aseismic since at least 1932. Although much of this area has been characterized by crustal mobility since at least the turn the century, the described uplift seems to be an unusually large and probably unique event superimposed the existing pattern of continuing deformation.

The results of levelings along U.S. Highway 395 between Lee Vining and Toms Place, California, in 1957, 1975, 1980, 1982, and 1983 define the development of a dome‐shaped uplift across the Long Valley caldera. These levelings indicate that Toms Place remained virtually invariant with respect to Lee Vining during the full interval 1957–1983, implying that the end points of the survey route lie beyond the range of the developing dome. No significant uplift occurred before 1975, but by October 1980 the uplift near Casa Diablo Hot Springs reached 0.25 m and the half width of the dome was of the order of 25 km. The uplift of Casa Diablo Hot Springs increased to 0.35 m by August 1982 and to about 0.40 m by August 1983 without any appreciable increase in the lateral extent of the dome. The August 1982 to August 1983 uplift apparently occurred before February 1983 with little or no uplift during the period February–August 1983. The 1975–1983 uplift is attributed to the injection of 0.19 km3 of magma into a spherical chamber at a depth of 10 km beneath the southern quadrant of the ancient resurgent dome within the caldera. Examination of the full vertical control record through the Long Valley area indicates that the 1975–1983 uplift is unique within the period 1905–1983. Although comparisons of the results of a 1932 leveling against a 1914 datum disclose a 1914–1932 down‐to‐the‐northwest tilt of about 10 µrad over the reach between Toms Place and Lee Vining, this tilt (if real) is more reasonably attributed to regional tectonic deformation than it is to the movement of magma at depth.

No abstract