Surfer 7 Grid File Format

(The really useful stuff is the "example" table and sample code at the very bottom.)

Surfer 7 grid files [.GRD] use a tag-based binary format to allow for future enhancements. Each section is preceded by a tag structure, which indicates the type and size of the following data. If a program does not understand or want a particular type of data, it can read the associated tag and quickly skip to the next section. In general, sections can appear in any order except for the first, which must be a Header section.

Data types used in Surfer 7 grid files:

Type	Description
long	32 bit signed integer
double	64 bit double precision floating point value

Each section is preceded by a tag structure with the following format:

Element	Type	Description
Id	long	The type of data in the following section. See the next table for a list of valid values.
Size	long	The number of bytes in the section (not including this tag). Skipping this many bytes after reading the tag will align the file pointer on the next tag.

Tag Id values. The 0x prefix indicates a hexadecimal value:

Id	Description
0x42525344	Header section – must be the first section within the file.
0x44495247	Grid section – describes a 2D matrix of Z values.
0x41544144	Data section – contains a variable amount of data. The size of the data section is given by the Size field in the tag structure.
0x49544c46	Fault Info section – describes the fault traces used when creating the grid.

The Header section must be the first section in the file and has the following format:

Element	Type	Description
Version	long	Version number of the file format. Currently must be set to 1.

The Grid section consists of a header that describes a 2D matrix of values, followed by the matrix itself. This section encapsulates all of the data that was traditionally referred to as a grid:

Element	Type	Description
nRow	long	number of rows in the grid
nCol	long	number of columns in the grid
xLL	double	X coordinate of the lower left corner of the grid
yLL	double	Y coordinate of the lower left corner of the grid
xSize	double	spacing between adjacent nodes in the X direction (between columns)
ySize	double	spacing between adjacent nodes in the Y direction (between rows)
zMin	double	minimum Z value within the grid
zMax	double	maximum Z value within the grid
Rotation	double	not currently used
BlankValue	double	nodes are blanked if greater or equal to this value

A Data section containing the 2D matrix of values (doubles) must immediately follow a Grid section. Within the Data section, the grid is stored in row-major order, with the lowest row (minimum Y) first

A Fault Info section describes the fault geometry used to create the grid. Fault Info sections have the following format:

Element	Type	Description
nTraces	long	number of fault traces (polylines)
nVertices	long	total number of vertices in all the traces
data section		variable-sized data block consisting of an array of Trace structures immediately followed by the array of vertices

A Data section containing an array of Trace structures and an array of Vertex structures must immediately follow a Fault Info section. The number of Trace structures in the array is nTraces , and the number of Vertex structures is nVertices .

Trace structure:

Element	Type	Description
iFirst	long	0-based index into the vertex array for the first vertex of this trace
nPts	long	number of vertices in this trace

Vertex structure:

Element	Type	Description
x	double	X coordinate of the vertex
y	double	Y coordinate of the vertex

Example

The following example illustrates the layout for a 5 row by 10 column grid (byte position is base 1):

Element	Byte Pos.	Type	Description
0x42525344	1	long	Tag: Id for Header section
4	5	long	Tag: Size of Header section
1	9	long	Header Section: Version
0x44495247	13	long	Tag: ID indicating a grid section
72	17	long	Tag: Length in bytes of the grid section
5	21	long	Grid Section: nRow
10	25	long	Grid Section: nCol
0.0	29	double	Grid Section: xLL
0.0	37	double	Grid Section: yLL
1.0	45	double	Grid Section: xSize
1.75	53	double	Grid Section: ySize
25.0	61	double	Grid Section: zMin
101.6	69	double	Grid Section: zMax
0.0	77	double	Grid Section: Rotation
1.70141e38	85	double	Grid Section: BlankValue
0x41544144	93	long	Tag: ID indicating a data section
400	97	long	Tag: Length in bytes of the data section (5 rows x 10 columns x 8 bytes per double)
Z1, Z2, …	101,109... (step 8 bytes to end of file)...	double	Grid data. First value is the lower left node, and the subsequent nodes procede across the row until the end, then up to the second row, etc., until finishing at the upper right node.

Sample Code

Use the example table to determine what data type you should use for each header value. For example, if you want to get the xMin value of the grid in Scripter, you would insert the following code:

Or, if you wanted to load all of the data into an array, you would use this:

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