# Surface Charge Density (.OL)

The data structure for the .ol files are described to understand and correctly post-process these files externally.

The following fields are available in the header block:

Table 1. Fields in the Header Block of the .ol file.
Key Required Description
File Type Yes Describes the type of the file. For surface charge density, the value must be Charges.
File Format Yes Denotes the file syntax version (such as “4”). If not present it defaults to version 1 (files pre-dating Feko 6.1).
Source Optional Denotes the base filename of the source where this data comes from.
Date Optional Date and time of data export in format “YYYY-MM-DD hh:mm:ss” (that is 24-hour format).

The following fields are available in the solution block header:

Table 2. Available fields in the solution block header of the .ol file.
Key Required Description
Configuration Name Optional The configuration name, if present.
Request Name Optional The explicit name given to that solution request (as denoted in the .pre file). If none is specified, POSTFEKO will use a default name of “request_N” (where “_N” is replaced with a number for each unnamed request) during import.
Frequency Yes Frequency in Hz for which the following data was measured/computed.
No. of [$$] Samples Yes The number of samples in each axis direction. The “[$$\$]” term is replaced by the following:
• Electric Charge Triangle
• Magnetic Charge Triangle
• Segment Charge
Spatial Units Optional Specify the units in which the position is defined.
Result Units Optional Specify the units in which the results are given.
No. of Header Lines Optional Number of header lines to read. The column header lines must follow this line. If this value is not specified, assume a value of “1”.
Yes For header lines, the following format should be used:
#No. of Header Lines: M
#"Column 1: Line 1" "Column 2: Line 1" ... "Column N: Line 1"
#"Column 1: Line 2" "Column 2: Line 2" ... "Column N: Line 2"
...
#"Column 1: Line M" "Column 2: Line M" ... "Column N: Line M"
For the column headers of charges, the only difference between triangles and segments is an optional additional field that stores the total surface area (for triangles) or the total length (for segments).
Triangles:
Num X Y Z Re(Q) Im(Q) [optional] Surface Area
Segments:
Num X Y Z Re(Q) Im(Q) [optional] Length