arc gis期末复习考点第八章.doc

Chapter 8 Review Questions1. Explain the difference between location errors and topological errors.Location errors such as missing polygons or distorted lines relate to the geometric inaccuracies of spatial features, whereas topological errors such as dangling lines and unclosed polygons relate to the logical inconsistencies between spatial features.2. What are the primary data sources for digitizing?Global positioning systems (GPS) and remote sensing imagery provide the primary data sources for digitizing. These data sources can bypass printed maps and the practice of various methods of map generalization. 3. A digitized map from a secondary data source such as a USGS quadrangle map is subject to more location errors than a primary data source. Why?A secondary data source such as a USGS quadrangle map is subject to more location errors because the map has undergone simplification, generalization, and other practices during the mapmaking process.4. Although the U.S. National Map Accuracy Standard adopted in 1947 is still printed on USGS quadrangle maps, the standard is not really applicable to GIS data. Why?A GIS uses digital spatial data, which can be easily manipulated and output to any scale. The U.S. National Map Accuracy Standard, on the other hand, is scale dependent. 5. According to the new National Standard for Spatial Data Accuracy, a geospatial data producer is encouraged to report a RMS statistic associated with a data set. In general terms, how does one interpret and use the RMS statistic?One can first multiply the RMS by 1.7308 to get the NSSDA (National Standard for Spatial Data Accuracy) statistic. This statistic represents the standard error of the mean at the 95 percent confidence level. In other words, one can be sure that, 95 percent of the time, the accuracy of a point or a line is within the NSSDA statistic.6. Suppose a point location is recorded as (575729.0, 5228382) in data set 1 and (575729.64, 5228382.11) in data set 2. Which data set has a higher data precision? In practical terms, what does the difference in data precision in this case mean?Data set 2 has a higher data precision than data set 1. If the measurement unit is meters, the recording of a point location in data set 2 is down to one hundredth of a meter and the recording in data set 1 is rounded off at meters. 7. The ArcGIS Desktop Help has a poster illustrating topology rules in the geodatabase data model (ArcGIS Desktop Help Editing in ArcMap Editing Topology Topology rules). View the poster. Can you think of an example (other than those on the poster) that can use the polygon rule of “Must be covered by feature class of”?The poster illustrates the polygon rule with the example of “States are covered by counties.” By extension, counties must be covered by census tracts, census tracts by block groups, and block groups by blocks.8. Give an example (other than those on the poster) that can use the polygon rule of “Must not overlap with.”The poster illustrates the polygon rule with the example of “Lakes and land parcels from two different feature classes must not overlap.” The Census Administrative Boundaries Data Model poster that can be downloaded from the ESRI website has two other examples: the feature class of American Indian Reservation must not overlap with the feature class of Place:city, and the feature class of American Indian Reservation must not overlap with the feature class of Place:town.9. Give an example (other than those on the poster) that can use the line rule of “Must not intersect or touch interior.”The poster illustrates the line rule with the example of “Lot lines cannot intersect or overlap and must connect to one another only at the endpoint of each line feature.” Like lot lines, road center lines cannot intersect or overlap.10. Use a diagram to illustrate how a large nodesnap for editing can alter the shape of line features.Node a is supposed to be snapped to node b. But a large nodesnap can snap node a to node c instead. 11. Use a diagram to illustrate how a large cluster tolerance for editing can alter the shape of line features.A large cluster tolerance can incorrectly snap the two lines in the center of the diagram indicating a small stream channel. 12. Explain the difference between a dangling node and a pseudo node.A dangling node is at the end of a dangling arc, whereas a pseudo node appears along a continuous line and divides the line unnecessarily into separate lines.13. What is a map topology?A map topology is a temporary set of topological relationships between the parts of features that are supposed to be coincident.14. Describe the three basic steps in using a topology rule.Step 1: create a new topology by defining the participating feature classes, the ranks for each feature class, the topology rule(s), and a cluster tolerance. Step 2: evaluate the topology rule and creates errors indicating those features that have violated the topology rule. Step 3: fix errors or accepts errors as exceptions.15. Some nontopological editing operations can create features from existing features. Give two examples of such operations.Examples include merge features, buffer features, union features, and intersect features.16. Edgematching requires a source layer and a target layer. Explain the difference between these two types of layers.Features, typically vertices, on the source layer are moved to match those on the target layer