This LandView 5 program function produces total population and housing unit counts for a user-specified radius. If you are trying to determine additional demographic characteristics or the populations residing in an area not defined by a circle see Alternate Population Search Methodologies.How LandView 5 Creates the Population and Housing Estimate
The population and housing unit estimate is created by tallying Census 2000 block data for those block internal points (i.e. calculated 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 area for which we 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 Census 2000 Population and Housing Summary File 1 data, involving mo
re than 8 million census blocks nationwide. To conserve file space and improve mapping performance, one-third of the 8.2 million Census Blocks that do not contain either any population or housing units are excluded from the LandView 5 Census 2000 database and are not shown on the maps.
NOTE 1: This function, uses Census 2000 block points to provide the most precise estimate for small areas (i.e. radii of 1-3 miles). Although it can be used for larger radii, processing time is significantly increased as the size of the search circle increases. Because there are more than 5.4 million census block points in the entire U.S., the LandView program will be overloaded and STOP when trying to process a very large radius (100 miles or greater) in a densely populated area of the country like New York City Area. For this reason, the population estimator will change the basis of this calculation to census tract points for distances greater than 25 miles.
Nationally, there are over 66,000 census tract points. Although census tracts are much larger in area than census blocks, the loss of precision is negligible when calculating population living within a large area. The user can change the 25 mile default to some other value in the screen displayed by the "Instructions for using this estimator" button.
NOTE 2: The estimator will tally only block point records for states found on the CD-ROM or the DVD. Therefore the user should map the radius to see whether the radius extends beyond one or more state(s) boundaries by using the Show this radius on map function. The census tract point file included with the population estimator on the state CD-ROMs and the national DVD's includes census tract centroid data for all states, the District of Columbia and Puerto Rico. Consequently, the results for the census tract will reflect the population living within census tracts encompassed by the circle. The census tract point file included with the Prince William County, VA demo file is limited to the District of Columbia, Maryland and Virginia.
You can access the population estimator function by clicking the Population Within Radius button on the LandView 5 Startup screen or from MARPLOTŪ by clicking Sharing - LandView - LandView Census 2000 Population Estimator. Follow the three steps provided in the Instructions for using this 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 in the Population Estimator, or calculates a new population estimate for a different set of coordinates.
The shaded area resides on a layer called 'LV work layer' in the [Other] group. This is a temporary layer. If you wish to save this for reference, create a new layer and move the map object to the layer prior to shutting down MARPLOT. Details on how to do this are available in MARPLOT Help and in the Users Manuals.
Note 3: The LandView program's algorithm for determining which block internal points fall within the radius takes the curvature of the earth into account. The MARPLOT mapping engine in mapping the radius does not. Consequently for larger radii, users might note differences in the block point counts between MARPLOT and the Population Estimator.
