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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:

Dim xMin As Double

Open [your filename] For Binary As #1

Get #1,29,xMin

Close #1

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

Dim xCols As Long

Dim yRows As Long

Dim myData() As Double

Open [your filename] For Binary As #1

Get #1,5,yRows

Get #1,10,xCols

ReDim myData((xCols * yRows) -1)

Get #1,101,myData

Close #1

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