A data model describes the thematic layers used in the application (for example, hamburger stands, roads, and counties) their spatial representation (for example, point, line, or polygon) their attributes their integrity rules and relationships (for example, counties must nest within states) their cartographic portrayal and their metadata requirements. In ArcGIS, a set of database design specifications for objects in a GIS application. For example, the vector data model represents geography as collections of points, lines, and polygons the raster data model represents geography as cell matrixes that store numeric values and the TIN data model represents geography as sets of contiguous, nonoverlapping triangles. In GIS, a mathematical construct for representing geographic objects or surfaces as data.
The other two methods are the transit rule and the compass rule. In ArcMap and Survey Analyst for field measurements, one of three adjustment methods available for adjusting closure error for a traverse computation. In some circumstances the results of this adjustment method may be unexpected, or the adjustment may not be possible, and an alternative method is required. This method uses a least-squares adjustment to distribute the closure error, and applies infinite weight to the angles or direction measurements to ensure that they are not adjusted. It assumes that course directions and angles have no error and, therefore, all error corrections are applied only to the distances.
The Crandall rule is most frequently used in a closed traverse that represents a parcel from a subdivision plan to ensure that tangency between courses remains intact as, for example, when applied to a tangent curve. The other two methods are the transit rule and the Crandall rule.Ī special-case, least-squares-based method for adjusting the closure error in a traverse. The compass rule is also known as the Bowditch rule, named for the American mathematician and navigator Nathaniel Bowditch (1773-1838).
The correction factor at each point is multiplied by the overall closure error to get the amount of error correction distributed to the point's coordinates. More specifically, a correction factor is computed for each point as the sum of the distances along the traverse from the first point to the point in question, divided by the total length of the traverse. This rule distributes the closure error over the whole traverse by changing the northings and eastings of each traverse point in proportion to the distance from the beginning of the traverse. A widely used rule for adjusting a traverse that assumes the precision in angles or directions is equivalent to the precision in distances.
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