Surpop V2.0: Output file formats

The surface models accessed by Surpop are held as a series of georeferenced grids, and may be saved in a variety of different formats suitable for the user's own programs or GIS software.

Two broad categories of output file are available: those which contain records for only the non-zero cells in the specified output area, and those which contain records for all cells, including those with value zero. The former are generally much smaller, but the latter are more often required by raster GIS software. An optional header may be added to output files giving a basic description of the data which follow.

Non-zero cells only

Two options are available which select only the cells containing non-zero values. The first of these is a file containing three columns of values separated by spaces. The values represent the row, column and cell value for each non-zero cell, and the file extension .rcv is used for this type of output. Row and column are relative to the selected region, With the NW corner being position 1,1. When large regions are extracted, the cell records may not necessarily follow in strict geographical sequence, but will be structured according to the 100km grid sheets used in the creation of the national datasets. This file format provides compatibility with earlier surface handling programs described in Martin (1990). The output file looks like this:

Row Column Cell value
1 15 26.5 ie. row 1, cell 15 contains a value of 26.5
1 16 13.0
2 15 10.0
etc.

The second non-zero only output format contains X and Y coordinates for each cell centre, followed by its value. This format is suitable for input to Arc/Info as point data or to UNIRAS. The file extension is .xyz. The data are again in row primary order, beginning at the top left. The output file looks like this:

X value Y value Cell value
100100 150100 26.5 ie. the cell centred on 100100, 150100 contains a value of 26.5
100300 150100 13.0
100100 149900 10.0
etc.

The size of non-zero cells only output will be dependent on the number of non-zero cells in the selected region. This means that it is difficult to estimate the size of file that will be created, without prior knowledge of its population density. However, as a guide a 100km window centred on London would contain about 50,000 data records (cells) and one based on Bristol about 20,000. Smaller windows, or less populated areas will create much smaller files of data.

Full matrix formats

Three output options write files containing a value for every cell in the matrix, where a cell's location is determined by its position in the output file.

The first full matrix output is suitable for input to the IDRISI GIS, and uses the .img file extension. This file contains one record for each cell value in the selected region, in row primary order, starting at the cell in the NW corner. The 'Raster (from bottom left)' option uses the .rsb extension and creates a file identical to the .img option, but starting from the cell in the SW corner. In both cases, the files contain a single value for each record:
.img format
0 top left cell contains 0
34.0 next cell to the right contains 34.0
23.0 etc.

The final full matrix format writes a file suitable for input to the GRID module of Arc/Info, and writes one record for each cell value, starting in the NW corner, but with the addition of a 5-line header as follows:
Header contains Explanation
NCOL 100 100 columns of data
NROW 100 100 rows of data
XLLCORNER 455000 X reference of SW corner of data (metres)
YLLCORNER 105000 Y reference of SW corner of data (metres)
CELLSIZE 200 cell size (metres)

Users are strongly recommended to read the relevant documentation when transferring data into IDRISI or Arc/Info. See the DOCUMENT or the ASCIIGRID commands in the respective manuals. Note that both of these systems provide useful facilities for building larger geographical coverages, or for 'windowing' to smaller areas for analysis.

Optional 12-line header

An optional 12-line header may be added to any output format except the Arc/Info grid format, which has an Arc/Info-specific header. The content of the 12-line header is as follows:

Record number Contents
1 Descriptive text 1: "Surpop v2.0, source=input filename"
2 Descriptive text 2: output file format
3 X reference of SW corner of data (metres)
4 Y reference of SW corner of data (metres)
5 X reference of NE corner of data (metres)
6 Y reference of NE corner of data (metres)
7 cell size (metres)
8 minimum of non-zero data records
9 maximum of non-zero data records
10 mean of non-zero data records
11 0.0 (not used)
12 number of non-zero data records to follow

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Last modified: 18 July 1997