Uncertainty in Data

Descriptive data seldom specify a location with absolute precision and certainty. This notion introduces variable levels of uncertainty for calculated results that are based on spatial information extracted from the descriptions. Uncertainty must be identified, classified, and quantified in order that results may be used appropriately for spatial analysis or display purposes. The following section introduces and discusses types and sources of uncertainty inherent in the descriptions themselves and in geographic reference data for described places.

2.1.5.1 Descriptive Data

As discussed in Section 2.1.4.2 above, location descriptions convey the location of the collection locality relative to known geographic features. An analogy to a journey is used here for considering traverse type location descriptions. Named places or intersecting geographic features frequently designate the beginning of the journey with the collection locality at the end of the journey. The place where one begins the journey is specified in the description along with the distance, direction, and sometimes, the path (or route) to follow. The locality is at the end of the journey but the certainty of its spatial coordinates relies on the geometric and relational components that are provided in the spatial scene’s description. The definition of the start point and the precision of direction and distance values are most important.

Problems encountered with location descriptions include: the amount of detail that is provided with the description, the physical extent of location(s) referenced by the description, variable and unstated levels of precision for measurements, and use of secondary source material. The level of detail provided in the description is not proportional to the quantity of text but rather the horizontal extent of identifiable reference features and relative precision provided by the stated unit(s) of measure. The information contained in database descriptions of location is secondary source material because it is manually transcribed from primary (or other secondary) sources. This process can introduce error not

name used at difference locations may be returned from queries of reference sources.

2.1.5.2 Geographic Names

Geographic feature names may prove problematic when multiple features share a common name; the homonym problem. For example, there are 975 places named Beaver Creek or some variation, in United States. Site precision or locality precision levels of spatial resolution are desired from the description. The locality

description, “Pachalka Spr., Clark Mt.”, uses two place names to specify the

location. Clark Mountain covers most of the extent of one township (36 mi.2_{) and}

provides sub-regional precision level spatial resolution. Clark Mountain does however, provide good reference for the spring as it is inferred that the smaller spring is near the larger mountain. This would be useful for differentiating between springs if several are named Pachalka within the same county. Pachalka Spring provides site quality spatial resolution; it specifies a named natural feature (and perceived surrounding environment) that is unlikely to exceed more than a hectare or two. This extent approaches the one arc second precision of standard reference sources like the USGS’s GNIS gazetteer. The uncertainty in the description, Slate Range Mtns, between Death Valley and Borax Lake., is substantial; one feature of regional level spatial resolution is located by placing it between two adjacent features of regional extent. This location description requires that a point represent a region that may exceed 100 square miles.

2.1.5.3 Spatial Relation Variables

Descriptions include variable and unstated levels of precision for measurements of distance and direction. Distance measures are specified through a mix of English and metric units ranging in length from a foot to a mile and meter to kilometer. Measure quantities used with distances also vary widely in precision. Section 4.1.3.1 provides a discussion of normalization procedures for precision in variable distance units and quantities.

Distances described in the herpetology collection range from 2.5 yards to 63 miles. These are seldom recorded with precision greater than one place to the right of the decimal for any unit of measure. It is difficult to assess consistently, the level of precision for distance measures. It appears that distance measures are a mix of measured and estimated values. Distances measured with

automotive odometers are common and distinct; their descriptions refer to traveled routes and similar to most vehicle odometers, distances are recorded in miles with one place to the right of the decimal. Frequent estimation is apparent in other values listed. Distances in units of yards are illustrative of this

with few exceptions. Those values between 100 and 1000 are evenly divisible by 50 and values greater than 1000 are divisible by 100. It is apparent these values are not measured but estimates of perceived distance or roughly scaled from a base map.

Direction measure is described almost entirely by use of cardinal directions of unspecified declination. Horizontal direction is seldom recorded in units of angular measure and is typically specified in cardinal directions with common usage of first and second inter-cardinal directions. This translates to angular measure with a precision no greater than 22.5 degrees. There is no mention as to whether directions are true or magnetic; it is likely both are present. This

element of uncertainty by itself has the potential to skew second intercardinal directions one full interval (e.g. East Northeast to East) as the difference between magnetic and true north in the project study area (13 degrees – 20 minutes) is greater than the half the difference (11 degrees – 15 minutes) between the second intercardinal directions.

2.1.5.4 Location Variation with Time

Uncertainty due to temporal variation is significant for those collections with considerable time depth. The herpetology database includes collection records dating from the late 19th _{Century through the early 21}st _{Century. Significant}

change can occur in the name given to a place, the extent of the place, or its location. The greatest temporally induced spatial problem relates to changes in named places. Named places referenced by location descriptions can change in name or location over time. Consider the number of streets named for Dr. Martin Luther King, Jr. before 1968 and since that time. More pertinent to the collection database is the change of Joshua Tree National Monument to Joshua Tree National Park. Significant changes in the extent of named places introduce uncertainty as well. The locality description, 0.25 mi W Palm Springs, written in 1918 almost certainly refers to a location within the current city limits given population growth of that community over the past 86 years. Natural landmarks typically move on a geological time frame but cultural features can and do move in short order. The physical town site of Ruth, Nevada (including buildings) has been moved twice in the past century to accommodate expansion of mining areas. It is currently more than two miles from its previous location. Some places and routes cease to exist. Several neighboring communities of Ruth, Nevada are now within the operations area of the mine and no longer exist. In similar manner, the alignment of portions of the Interstate highway system covered or eliminated previously constructed and designated travel routes.