This LandView® IV program function produces a total population and housing unit count for a specified radius.
The estimate is created by tallying 1990 census block data for those block internal points (i.e. approximate centers) whose coordinates fall within the circle defined by the radius. The reason for using the Census Block internal point is that we do not have the latitude and longitude of every housing unit in our data base so we must use the next best available small geographic area for which we do have the tabulated population data, and for which we have a latitude and longitude. This means, that there will be cases where some Census Blocks will not be included in the tally even though part of the block, possibly with people living there, is within the radius, because their internal points fall outside of the radius. The opposite also occurs where the internal point of a Census Block is within the radius, but much of its land area, and population is outside. Generally speaking, on radii beyond a few miles, these "errors" cancel each other out. Note that this also means that some small radii, which do not include an internal point, will erroneously show a zero population. These demographic data are based on the 1990 Census of Population and Housing STF 1B data involving over 5 million census blocks nationwide. (Although there were over 7 million blocks for the 1990 Census, we have included only the 5 million census blocks that contain population or housing units.)
Please note that block internal points shown on the MARPLOT® map for the continental United States were computed under North American Datum 27 (NADS 27) for the 1990 Census while the features shown in the TIGER/Line® 1998 layer and Census Demographic layers were computed under NADS 83. Because of the coordinate differences between datums, the block internal points may appear off center when compared to these features. Alaska and Hawaii and other outlying areas outside of the continental United States are based upon local datums.
After the 1990 Census, Geography Division found some additional problems with positional accuracy of the source materials used to create the maps for Alaska. Subsequent to computation of the block internal points, roads and other features were adjusted to compensate for these problems. For these reasons, the block internal points in Alaska may appear off center.
You can access the population estimator function by clicking the Population Estimator button on the LandView Startup.LV4 menu or clicking Sharing - LandView - Estimate Population around current map pointer in MARPLOT. Then follow the four steps provided in the LandView 1990 Census Population Estimator.
After entering the radius and calculating the population and housing data, the Show this radius on map button will create a red shaded map object on the map. This shaded area will remain on the map until the user clicks the Clear all fields button or calculates a new population radius from a different location for a different coordinate.
If you wish to preserve these objects, you can do so by creating a user layer. Unlocking the layer allows you to copy the object to the layer and save it. For further details on how to do this check the MARPLOT Help and Users manual.
Although this function can be used for much larger radii, processing time is significantly increased as the size of the search circle increases. For these larger radii, users should use the MARPLOT List - Search commands to create a search collection of census tracts or counties. After creating the "Search Collection", the user can then have MARPLOT show all the found objects in the "Search Collection" by clicking the Show All on map button. The user would then click on the Sharing - LandView- Get Info menu item which then opens the appropriate LandView database and creates a subset of records marked on the map. The user can then click on File - Export Records from the LandView menu to copy the selected database records to a spreadsheet or database file for further analysis.
Source: U.S. Census Bureau
Last Revised: Tuesday, 20-Nov-2001 14:57:33 EST
