Here is the gnuplot macro to be used in the gnuplot call:
set terminal wxt size 1000,700
set title "EH Iso at " . strftime("%Y-%m-%d %H:%M:%S",time(0)+3600)
#set view 0, 0, 1.4, 1
set contour
set isosamples 40
#unset surface
set surface
splot "iso.dat" with lines
Here is the source code for the rather simple C-program:
// module makeGnuplotData
// Generates data for subsequent call of gnuplot
// E.Huckert 02-2022
// 03-2022: this version is OK (tested under Windows 7 and Linux)
// Compile (GNU gcc under Linux):
// gcc -o gmakeGuplotData makeGnuplotData.c -lm
// For isolines (contour) in gnuplot:
// Produce the data, put them in file iso.dat
// makeGnuplotData > iso.dat
// Write a very basic macro (name=iso.gnu)for gnuplot:
/*
set xrange [0:13]
set yrange [0:15]
set zrange [0:10]
set contour
unset surface // optional, use # to comment out
splot "iso.dat" with lines
*/
// Call then gnuplot as follows:
// gnuplot iso.gnu -p
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
// --------------------------------------------
void usage()
{
fprintf(stderr, "call: makeGnuplotData >outfile\n");
fprintf(stderr, "E.Huckert 03-2023\n");
}
// --------------------------------------------
void drawHLine(int x1, int x2, int y, int z)
{
printf("%d %d %d\n", x1, y, z);
printf("%d %d %d\n", x2, y, z);
} // end drawHLine()
// --------------------------------------------
void drawVLine(int x, int y1, int y2, int z)
{
printf("%d %d %d\n", x, y1, z);
printf("%d %d %d\n", x, y2, z);
} // end drawVLine()
// --------------------------------------------
int main(int argc, char *argv[])
{
int x = 1;
int y = 1;
int xOffs = 1;
int yOffs = 2;
int yLen = 10;
int xLen = 10;
int zv = 2;
int x1, x2, y1, y2;
// Generate GRID data for gnuplot (output should be catched via redirection)
// Draw some 3-dimensional (roof like) objects
x1 = x + xOffs;
x2 = x1 + xLen;
y1 = y + yOffs;
y2 = y1 + yLen;
//
for (int n=0; n < 3; n++)
{
drawHLine(x1, x2, y1, zv);
drawVLine(x2, y1, y2, zv);
drawHLine(x2, x1, y2, zv);
drawVLine(x1, y2, y1, zv);
printf("\n");
//
x1 = x1 + xOffs;
x2 = x2 - xOffs;
y1 = y1 + yOffs;
y2 = y2 - yOffs;
zv = zv + 2;
} // end for int ...
//
zurueck: // label not used
return 1;
} // end main()
Case 2: Contour Plots using real metereology data
This scenario is a little bit more complex than the preceeding one: it uses real metereological data from DWD (Deutscher Wetterdienst). You may find the data used here and many similar data sets under this link. There are lots of other data provided by DWD - not all given in the simple ASCII format that we use here.
The procedure works as follows:- Get real metereological data (here: half-year precipitation for Germany) from DWD. For easier use (the original file names are really long!) I renamed the file.
- Decompress the file using unzip or something similar.
- Use my C Program called prepareDWDData to rearrange the data (details are below). This program is portable between Windows and Linux.
- Start gnuplot with a macro (iso2.gnu) that reads the rearranged data and produces a image showing the contours for regions having similar precipitation.
The C program is needed as gnuplot expects 3-dimensional data in a special form: after each row an empty line must be inserted. If you look at the ASCII (non-binary) data provided by DWD you will see that the file contains only precipitation sums for each cell in a rectangular area. A value of -999 indicates that no data are found at this grid cell (which is the probably outside the region of interest).
The program reads the first 6 lines (header) from the data file. These lines contain meta information for the data, among them
- NROWS: the number of rows for the area
- NCOLS: the number of rows for the area
- CELLSIZE: the cell width (not considered here)
