Creating and Translating Grid Files for SURFER and ArcGIS:
SURFER Format:
If you have a data file that already contains gridded data, the following header can be placed in the file and the file directly imported into SURFER:
id
the code letters DSAA which identify the file as an ASCII grid file.
(note: DSAA must be in caps)
nx ny
nx (integer) is the # of grid lines along the x axis (columns)
ny (integer) is the # of grid lines along the y axis (rows)
xlo xhi xlo min. x value
of the grid
xhi max. x value of the grid
ylo yhi ylo min. y value
of the grid
yhi max. y value of the grid
zlo zhi zlo min. z value
of the grid
zhi max. z value of the grid
grid row 1 z data.............................................
grid row 2 z data.............................................
etc.
these are the z values of the grid, organized in row order. The grid row 1
corresponds to ylo and the last to yhi. (This means that the first line
corresponds to the “south” end of the map). As long as the grid values in the
file are ordered correctly from left to right, it does not matter how many
entries there are per line. Thus, you can put a grid row on one or more lines,
as long as the values are ordered consecutively.
For an example of the header, say you have generated a grid containing 11 rows and 6 columns. Edit the file by typing in the first five lines as described above (shown in bold italics):
DSAA
6 11
0 500
0 1000
100.0 104.0
100.0
100.9
101.3
101.5
101.6 101.5
100.9
101.3
101.5
101.7
101.8 101.8
101.6
101.7 101.9
102.0
102.0 102.0
102.1
102.1
102.2
102.3
102.3 102.3
102.5
102.5
102.6
102.6
102.6 102.6
102.8
102.9
103.2
102.9
102.8 102.8
103.0 103.1
103.2
103.1
103.1 103.0
103.2
103.2
103.3
103.2
103.2 103.2
103.4
103.4
103.4
103.4
103.4 103.4
103.5
103.5
103.5
103.5
103.6 103.7
103.6
103.7
103.6
103.6
103.7 104.0
download the file: example.grd
Save the file on disk. Next, once in SURFER, click on “MAP” and then “CONTOUR.” Find the file with the browse button and click OK. The next window will let you select the way you want to style your map. Click OK and the contour map should appear on the screen.
Note: you will probably want your grid to be mapped so that the first line
of the file will be the “north” row and the last line the “south” row. The row
order can be reversed in the file editor or you can do this directly in SURFER
(version 6 or higher). Choose GRID and then TRANSFORM. Select your grid file
and then choose “Mirror Y” as the operation. The new file (by default called
“OUT.GRD”) will be oriented properly and can then be contoured as needed. Here
is the map using default SURFER's default parameters:
ArcGIS Format:
ASCII grid files in ArcGIS have a header that is very similar to SURFER. By
swapping headers, you can convert an ArcGIS ascii grid file to SURFER and
vice-versa. The ASCII file must consist of header information containing a set
of keywords, followed by cell values in row-major order. The file format is
NCOLS ### (number of columns)
NROWS ### (number of rows)
XLLCORNER ### (this is the world x-coordinate
for the lower left corner of the map)
YLLCORNER ### (this is the world y-coordinate
for the lower left corner of the map)
CELLSIZE ### (edge length of the
raster cell – note that cells must be square)
NODATA_VALUE ### (value of “no data” so that areas without
data can be blanked on the map)
row 1 (begin the data, which is for
the most part free format)
row 2
.
.
.
row n
where ### is a number, and the
keyword NODATA_VALUE is optional and defaults to -9999. Row 1 of the data is at
the top of the raster, row 2 is under row 1, and so on. Using the same example
as the one above:
NCOLS 6
NROWS 11
XLLCORNER 0.0
YLLCORNER 0.0
CELLSIZE 100.0
NODATA_VALUE -9999
100.0
100.9
101.3
101.5
101.6 101.5
100.9
101.3
101.5
101.7
101.8 101.8
……
……
103.6
103.7
103.6
103.6
103.7 104.0
The NODATA_VALUE is the value in the ASCII file assigned to those cells whose true value is unknown. In the raster, they will be assigned the keyword NODATA. Cell values should be delimited by spaces. No carriage returns are necessary at the end of each row in the raster. The number of columns in the header determines when a new row begins. The number of cell values must be equal to the number of rows times the number of columns, or an error will be returned.
Once in this format, save the file
as *.asc and use the toolbox command <Conversion Tools> >> <To Raster> >> <ASCII to Raster> to translate the data
to ArcGIS grid format.