Tuesday, February 23, 2010

Pathloss 5.0 : GIS Setup (ch 3)

Digital Elevation and clutter Models

The following description is applicable to both digital elevation and clutter models. The term DEM (digital elevation model) is used for both elevations and clutter databases. A DEM is a regular array (rows and columns) of data. These rows and columns can be considered to form two different arrangements

· The rows and columns form an x-y axis grid. The data is defined at the intersections of the x and y grid lines.

· The rows and columns form rectangular cells. The data represents the average value in the cell area and is defined at the center of the cell.

The horizontal extent of the data is determined by the grid spacing or the cell size and the number of rows and columns. Suppose the xy grid spacing is one kilometer and the cell size is also one kilometer. A horizontal extent of 100 kilometers would require 101 rows / columns for grid referenced data and 100 rows / columns for cell referenced data.

The horizontal reference can be in geographic or projected coordinate system. For example the data base might be described as a 3 arc second in the case of a geographic reference or 30 meters in database using a UTM projection. In both cases the horizontal datum must be known.

Some DEMS use a file naming convention is used to identify the horizontal extents. In others, the spacial data is in For example the SRTM hgt files are in one degree by one degree blocks. The file name is based on the south west corner coordinates and the resolution of either 3 arc seconds or 1 arc second is determined by the size of the file. Other databases provide the spacial information in external files

The usual data format is an unsigned 16 bit integer. This can represent any elevation in meters relative to sea level or an index into a clutter definition table. Some DEMs will use 8 byte integers provided that the actual range of data values can be accommodated. Other DEMS use floating point numbers.

The actual data file contains only the elevations or clutter values and is written as a continuous sequence of data values. The file may contain a header describing the extents and resolution of the file or this information may be available in external files Several common formats are given below.

· data is written in columns from south to north starting at west edge and proceeding to the east edge

· data is written in rows from west to east starting at the south edge and proceeding to the north

· data is written in rows from west to east starting at the north edge and proceeding to the south edge

BIL file format

BIL ("Band Interleaved by Line) is a generic GIS data file format. A single band (elevation or clutter) is used in all databases in this program and this would be more correctly denoted by BSQ (Band Sequential). BIL files have the following characteristics.

· the data is referenced to the center of a cell

· data is organized in rows running from west to east. The origin can be in the south west or north west corner.

· data format can be 16 bit integer, 8 bit integer or IEEE floating point

· byte order can be big-endian or little-endian ( Motorola - Intel)

· the spacial reference can be geographic or a projection such as UTM. The file contains only the elevation or clutter data. An external file index is used to define the extents of each file.

· Planet data is an example of BIL projected data. The US NED is a an example of BIL geographic data.

File Locations

Other that the Site coordinates tab, each tab includes a Setup button which specifies the location of the files associated with that tab. These files can be located in individual directories or in a main or common directory. If a directory is specified for a file, the program first looks for the file in that directory. If the file is not found or the file directory is not specified, the program looks for the file in the main directory.

File Index

A file index specifies the extents and resolution of the data files. This is the lookup table which maps the coordinates of a point to the database file name. The mapping procedure checks for file with overlapping areas and uses the file with the higest resolution. The File index uses a standard grid data entry form for which details are provided in the general program operation section. The following fields are used:

· west, east, south, north edges

· number of rows and columns

· x and y cell sizes

· UTM zone - standard meridian this field is only used on projections which have been specified as variable

Either the cell sizes or the number of rows and columns can be specified. The other fields will be calculated. If only the x cell dimension is entered, it is assumed that the y cell is the same size.

All dimensions must correspond to the specified projection. For example a DTED file (geographic) would have the edges specified in degrees and the cell size in seconds as follows:

west edge = 105W, east edge = 104W, south edge = 51N, north edge = 52N, x cell = 6 (seconds) y cell = 3 (seconds)

UTM projected data would have all units expressed in meters as in the example line below

west edge = 535590.0 m, east edge =545590.0 m, south edge = 721330.0 m, north edge = 731330.0, x cell = 10m

mention temporary file index here

talk about how the index is used and always the best resolution

Import Methods

Select Files - Import index on the File index menu bar. A submenu of the available import methods is displayed. The following methods to import an index are used:

· if the data files include header information or uses a file naming convention, a standard windows file open dialog is use to multi select the files. The headers are read and the index is automatically generated. Examples of this method are DTED and SRTM hgt files

· ESRI Arc Info/Grid employ separate files with the suffixes hdr, blw, and prj files to define the extents and projection of the data file which usually has a bil suffix. These files can be used to automatically generate an index item for the file.

· A text file index is included in Planet type data sets. This file can be imported using the standard text import utility described in the general program operation section.

· The file index menu may include an item to convert an ASCII file to the binary DEM format. This procedure will automatically generate an index for the converted file.

DEM Parameters

Right click on an index item or select Edit - Parameters from the File index menu bar. The DEM Parameters dialog shows the default DEM configuration.

· data type - default value - do not change

· byte order - A change may be necessary when using the generic BIL projected and geographic formats.

· location - default value - do not change

· units - default value - do not change

· no data value - changes allowed based on specific data file information.

· treat no data values as sea level - the default value is false. Users can change this based on actual knowledge of the area.

· read data as.. The default setting are "4 point weighted average" on DEMs and single point on clutter data bases. If the DEM contains embedded building data, this should be set to a single point read

· file organization - default value - do not change

· file origin - on BIL data, this setting effectively turns the display upside down. A change may be necessary when using the generic BIL projected and geographic formats

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