Calculates the distance from points or polygons to network CENTERS.  The distance is calculated from each point or polygon centroid to the nearest road, and from that location to each network CENTER by way of the shortest path along the road network.  Results are written to the input coverage PAT.



NETDISTANCE <in_cover> <road_cover> <center_cover> {center_item} {bnd_cover} {cellsize} {selection_file}




<in_cover> - the input point or polygon coverage (e.g. timber stands.)


The distance from each input feature to each point (CENTER) in the <center_cover> is calculated by traveling to the nearest arc in the <road_cover> and then taking the shortest path to the CENTER.


If the <in_cover> is a polygon coverage, then the distance is calculated from the centroid of each polygon.


<road_cover> - the input line coverage of the road network along which distances are to be calculated.


<center_cover> - the input point coverage that identifies the CENTERS on the <road_cover> to which distances are to be calculated.


The CENTERS should fall on or near to one or more arcs in the <road_cover>.  In a temporary copy of the <center_cover> the CENTERS are snapped to nearest road, so long as they are within 1000 map units.


{center_item} - the item in the <center_cover> PAT that is used to name the output items in the <in_cover> PAT.


The items are named: DIST_<center_value>, where <center_value> is the value of the {center_item}.  An item is created for each different value.  Therefore the values should be unique.


If an item already exists, the values will be overwritten.


If no {center_item} is specified, it defaults to <center_cover>#.


{bnd_cover} - the coverage whose BND is used to define the extent of the analysis area.


The analysis area is extended 1000 map units beyond the BND of the {bnd_cover} on all sides.


If no coverage is specified, then the extent defaults to the area which encompasses the BNDs of both the <in_cover> and <center_cover>, and is extended 1000 map units on all sides.


{cellsize} - the size of the grid cells used to do the processing.


NETDISTANCE converts the road and center coverages into grids in order to do the analysis.  The smaller the {cellsize} the more accurate the results, but the longer the processing time.  In general the {cellsize} should be small enough so that neighboring roads that do not intersect still do not touch once they have been converted into grids.  Otherwise this will create shortcuts.


If no {cellsize} is specified, it defaults to 1/1000th of the maximum dimension of the analysis area.


{selection_file} - the ARCPLOT selection file that identifies which roads can be connected to directly from the input features.


If no selection file is specified, then it is assumed that all roads can be connected to directly.


Here's an example of the utility of this argument.  Say the <in_cover> is potential timber stands, the <center_cover> is mill locations, and NETDISTANCE is being used to approximate the haul distance from each stand to each mill.  However, some of the roads (e.g. highways) can be driven on but cannot be used for logging activities (e.g. building spur roads to the stands.)  So use the {selection_file} to identify all of the roads that can be used for logging activity.





NETDISTANCE runs from the ARC module of ArcInfo.





Arc: &run netdistance usage

Usage: NETDISTANCE <in_cover> <road_cover> <center_cover> {center_item} {bnd_cover} {cellsize} {selection_file}


1)  To calculate the approximate distance from timber stands to mills along a road system:


Arc: &run neardistance veg_cover road_cover mill_cover mill_name # 15


2)  To do the same thing, but specify that Interstate 84 can not be connected to directly:


Arc: arcplot

Arcplot: reselect road_cover arc id not cn `I-84'

Arcplot: writeselect not_i84

Arcplot: quit

Arc: &run neardistance veg_cover road_cover mill_cover mill_name # 15 ~

Arc: not_i84