[Bug] v.out.vtk: Replace Geometry Output datatype "float" by "double"

[ErikNixdorf]

Describe the bug
Geometric information in the VTK Polydata dataset output of the v.out.vtk function are saved as datatype "float". However, considering the amount of digits in geographic coordinates, reading these vtk files with other vtk readers (e.g. ParaView) can result in a lack of precision

Expected behavior
Replace standard output format "float" by "double" for the geometry fields in order to overcome this problem
System description (please complete the following information):

• Operating System: Windows
• GRASS GIS version 7.8.2

[BadAssassin]

Does this patch address the issue? I am not terribly familiar with VTK file format.

diff --git a/vector/v.out.vtk/main.c b/vector/v.out.vtk/main.c
index ebad7cbf1..5c5974201 100644
--- a/vector/v.out.vtk/main.c
+++ b/vector/v.out.vtk/main.c
@@ -68,7 +68,7 @@ int main(int argc, char *argv[])
     dp_opt->type = TYPE_INTEGER;
     dp_opt->required = NO;
     dp_opt->description =
-       _("Number of significant digits (floating point only)");
+       _("Number of significant digits");
 
     scale = G_define_option();
     scale->key = "zscale";
@@ -152,8 +152,8 @@ int main(int argc, char *argv[])
     if (dp_opt->answer) {
        if (sscanf(dp_opt->answer, "%d", &dp) != 1)
            G_fatal_error(_("Failed to interpret 'dp' parameter as an integer"));
-       if (dp > 8 || dp < 0)
-           G_fatal_error(_("dp has to be from 0 to 8"));
+       if (dp > 16 || dp < 0)
+           G_fatal_error(_("dp has to be from 0 to 16"));
     }
     else {
        dp = 8;                 /*This value is taken from the lib settings in G_feature_easting */
diff --git a/vector/v.out.vtk/writeVTK.c b/vector/v.out.vtk/writeVTK.c
index 8b8ba60eb..66cd5e66a 100644
--- a/vector/v.out.vtk/writeVTK.c
+++ b/vector/v.out.vtk/writeVTK.c
@@ -199,7 +199,10 @@ int write_vtk_points(FILE * ascii, struct Map_info *Map, VTKInfo * info,
     /*Write the coordinates into the vtk ascii file */
     /************************************************/
 
-    fprintf(ascii, "POINTS %i float\n", info->maxnumpoints);
+    if (dp > 8)
+        fprintf(ascii, "POINTS %i double\n", info->maxnumpoints);
+    else
+        fprintf(ascii, "POINTS %i float\n", info->maxnumpoints);
 
     /*For every available vector type */
     for (k = 0; k < typenum; k++) {

[ErikNixdorf]

to me, it looks good. However, I am a bit wondering why you dont just replace floatby double in any case. But as I said, the case-sensitive solution provided above is good, too

[BadAssassin]

Like I said... I'm not terribly familiar with the VTK file usage, so I don't know if there is a specific need for float, so I included it just in case.

I will submit a pull request.