Working with ZMAP+ grid files in Python

I just added to the awesome Geoscience I/O library a notebook showing how to use Python to read in an XYZ grid file in legacy ZMAP+ format as numpy array, display it with matplotlib, write it to a plain ASCII XYZ grid text file. I will use the DEM data for Mount St. Helens BEFORE the 1980 eruption, taken from Evan Bianco’s excellent notebook, which I converted to ZMAP+ (that part I will demonstrate in a future post / notebook).

You can get the full notebook on GitHub.

ZMAP plus files have a header section that looks something like this (this one comes from the DEM file I am using):

! -----------------------------------------
! ZMap ASCII grid file
! -----------------------------------------
@unnamed HEADER, GRID, 5
15, 1E+030, , 7, 1
468, 327, 0, 326, 0, 467
0. , 0. , 0.

where the rows starting with a ! symbol are comments, and the rows in between the two @ symbols store the information about the grid. In particular, the first two entries in the third line of this column are nrows and ncols, the number of rows and columns,  respectively.

The header section is followed by a data section of ncols blocks each containing nrows elements arranged in rows of 5 (given by the last number of the first row with the @ symbol.

OK, let’s get to it.

I break the task of reading in two parts: the first part is to gather from the header the information I will be using. I want from the section between the two @s:

  • the second entry in its second line – the null value
  • the first two entries in its third line – the number of rows and columns, as mentioned
  • the last four entries in its third line – the xmin, xmax, ymin, ymax, respectively

I am making the assumption that the number of commented rows may not be the same in all ZMAP+ files, hence I abandoned my first approach which was based on using getline with specific line numbers. Instead, I used:

  • an if statement to check for the line beginning with @ (usually followed by a grid name) but not @\n; then grab the next two lines
  • another if statement to check whether a line starts with !
  • a counter is increased by one for each line meeting either of the above conditions. This is used later on to skip all lines that do not have data
filename = '../data/Helens_before_XYZ_ZMAP_PLUS.dat'
count = 0
with open(filename) as h:
    for line in h:
        if line.startswith('!'): 
            count += 1
        if line.startswith('@') and not line.startswith('@\n'):
            count += 1
            hdr.append(next(h)); count += 1
            hdr.append(next(h)); count += 1
            count += 1

which gives:

>>> ['15, 1E+030, , 7, 1\n', '468, 327, 0, 326, 0, 467\n']

Next I use the line hdr = [x.strip('\n') for x in ','.join(hdr).split(',')] to remove the newline, and the next if / else statement to deal with the null values, if present:

if hdr[1] ==' ': null=np.nan
else: null = float(hdr[1])

Finally, I get the remainder of the information with:

xmin, xmax, ymin, ymax = [ float(x) for x in hdr[7:11]]
grid_rows, grid_cols = [int(y) for y in hdr[5:7]]

Which I can check with:

print(null, xmin, xmax, ymin, ymax, grid_rows, grid_cols)
>>> 1e+30 0.0 326.0 0.0 467.0 468 327

That’s it! I think there is room to come back and write something more efficient later on, but for now I am satisfied this is robust enough to handle headers.

The rest, reading in the data, is downhill, with a first loop to skip the 9 header lines, and a second one to read all elements in a single array:

with open(filename) as f:
    [next(f) for _ in range(count)]                
    for line in f:
        grid = (np.asarray([word for line in f for word in line.split()]).astype(float)).reshape(grid_cols, grid_rows).T


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