src/dem.c

   1 /*
   2  * viking -- GPS Data and Topo Analyzer, Explorer, and Manager
   3  *
   4  * Copyright (C) 2003-2008, Evan Battaglia <gtoevan@gmx.net>
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  19  *
  20  */
  21
  22 #ifdef HAVE_CONFIG_H
  23 #include "config.h"
  24 #endif
  25
  26 #include <stdio.h>
  27 #ifdef HAVE_STRING_H
  28 #include <string.h>
  29 #endif
  30 #include <glib.h>
  31 #ifdef HAVE_MATH_H
  32 #include <math.h>
  33 #endif
  34 #ifdef HAVE_STDLIB_H
  35 #include <stdlib.h>
  36 #endif
  37 #include <zlib.h>
  38 #include <sys/types.h>
  39 #include <sys/stat.h>
  40 #ifdef HAVE_UNISTD_H
  41 #include <unistd.h>
  42 #endif
  43
  44 #include <glib/gstdio.h>
  45 #include <glib/gi18n.h>
  46
  47 #include "dem.h"
  48 #include "file.h"
  49
  50 #define DEM_BLOCK_SIZE 1024
  51 #define GET_COLUMN(dem,n) ((VikDEMColumn *)g_ptr_array_index( (dem)->columns, (n) ))
  52
  53 static gboolean get_double_and_continue ( gchar **buffer, gdouble *tmp, gboolean warn )
  54 {
  55   gchar *endptr;
  56   *tmp = g_strtod(*buffer, &endptr);
  57   if ( endptr == NULL|| endptr == *buffer ) {
  58     if ( warn )
  59       g_warning(_("Invalid DEM"));
  60     return FALSE;
  61   }
  62   *buffer=endptr;
  63   return TRUE;
  64 }
  65
  66
  67 static gboolean get_int_and_continue ( gchar **buffer, gint *tmp, gboolean warn )
  68 {
  69   gchar *endptr;
  70   *tmp = strtol(*buffer, &endptr, 10);
  71   if ( endptr == NULL|| endptr == *buffer ) {
  72     if ( warn )
  73       g_warning(_("Invalid DEM"));
  74     return FALSE;
  75   }
  76   *buffer=endptr;
  77   return TRUE;
  78 }
  79
  80 static gboolean dem_parse_header ( gchar *buffer, VikDEM *dem )
  81 {
  82   gdouble val;
  83   gint int_val;
  84   guint i;
  85   gchar *tmp = buffer;
  86
  87   /* incomplete header */
  88   if ( strlen(buffer) != 1024 )
  89     return FALSE;
  90
  91   /* fix Fortran-style exponentiation 1.0D5 -> 1.0E5 */
  92   while (*tmp) {
  93     if ( *tmp=='D')
  94       *tmp='E';
  95     tmp++;
  96   }
  97
  98   /* skip name */
  99   buffer += 149;
 100
 101   /* "DEM level code, pattern code, palaimetric reference system code" -- skip */
 102   get_int_and_continue(&buffer, &int_val, TRUE);
 103   get_int_and_continue(&buffer, &int_val, TRUE);
 104   get_int_and_continue(&buffer, &int_val, TRUE);
 105
 106   /* zone */
 107   get_int_and_continue(&buffer, &int_val, TRUE);
 108   dem->utm_zone = int_val;
 109   /* TODO -- southern or northern hemisphere?! */
 110   dem->utm_letter = 'N';
 111
 112   /* skip numbers 5-19  */
 113   for ( i = 0; i < 15; i++ ) {
 114     if ( ! get_double_and_continue(&buffer, &val, FALSE) ) {
 115       g_warning (_("Invalid DEM header"));
 116       return FALSE;
 117     }
 118   }
 119
 120   /* number 20 -- horizontal unit code (utm/ll) */
 121   get_double_and_continue(&buffer, &val, TRUE);
 122   dem->horiz_units = val;
 123   get_double_and_continue(&buffer, &val, TRUE);
 124   /* dem->orig_vert_units = val; now done below */
 125
 126   /* TODO: do this for real. these are only for 1:24k and 1:250k USGS */
 127   if ( dem->horiz_units == VIK_DEM_HORIZ_UTM_METERS ) {
 128     dem->east_scale = 10.0; /* meters */
 129     dem->north_scale = 10.0;
 130     dem->orig_vert_units = VIK_DEM_VERT_DECIMETERS;
 131   } else {
 132     dem->east_scale = 3.0; /* arcseconds */
 133     dem->north_scale = 3.0;
 134     dem->orig_vert_units = VIK_DEM_VERT_METERS;
 135   }
 136
 137   /* skip next */
 138   get_double_and_continue(&buffer, &val, TRUE);
 139
 140   /* now we get the four corner points. record the min and max. */
 141   get_double_and_continue(&buffer, &val, TRUE);
 142   dem->min_east = dem->max_east = val;
 143   get_double_and_continue(&buffer, &val, TRUE);
 144   dem->min_north = dem->max_north = val;
 145
 146   for ( i = 0; i < 3; i++ ) {
 147     get_double_and_continue(&buffer, &val, TRUE);
 148     if ( val < dem->min_east ) dem->min_east = val;
 149     if ( val > dem->max_east ) dem->max_east = val;
 150     get_double_and_continue(&buffer, &val, TRUE);
 151     if ( val < dem->min_north ) dem->min_north = val;
 152     if ( val > dem->max_north ) dem->max_north = val;
 153   }
 154
 155   return TRUE;
 156 }
 157
 158 static void dem_parse_block_as_cont ( gchar *buffer, VikDEM *dem, gint *cur_column, gint *cur_row )
 159 {
 160   gint tmp;
 161   while ( *cur_row < GET_COLUMN(dem, *cur_column)->n_points ) {
 162     if ( get_int_and_continue(&buffer, &tmp,FALSE) ) {
 163       if ( dem->orig_vert_units == VIK_DEM_VERT_DECIMETERS )
 164         GET_COLUMN(dem, *cur_column)->points[*cur_row] = (gint16) (tmp / 10);
 165       else
 166         GET_COLUMN(dem, *cur_column)->points[*cur_row] = (gint16) tmp;
 167     } else
 168       return;
 169     (*cur_row)++;
 170   }
 171   *cur_row = -1; /* expecting new column */
 172 }
 173
 174 static void dem_parse_block_as_header ( gchar *buffer, VikDEM *dem, gint *cur_column, gint *cur_row )
 175 {
 176   guint n_rows;
 177   gint i;
 178   gdouble east_west, south;
 179   gdouble tmp;
 180
 181   /* 1 x n_rows 1 east_west south x x x DATA */
 182
 183   if ( (!get_double_and_continue(&buffer, &tmp, TRUE)) || tmp != 1 ) {
 184     g_warning(_("Incorrect DEM Class B record: expected 1"));
 185     return;
 186   }
 187
 188   /* don't need this */
 189   if ( !get_double_and_continue(&buffer, &tmp, TRUE ) ) return;
 190
 191   /* n_rows */
 192   if ( !get_double_and_continue(&buffer, &tmp, TRUE ) )
 193     return;
 194   n_rows = (guint) tmp;
 195
 196   if ( (!get_double_and_continue(&buffer, &tmp, TRUE)) || tmp != 1 ) {
 197     g_warning(_("Incorrect DEM Class B record: expected 1"));
 198     return;
 199   }
 200
 201   if ( !get_double_and_continue(&buffer, &east_west, TRUE) )
 202     return;
 203   if ( !get_double_and_continue(&buffer, &south, TRUE) )
 204     return;
 205
 206   /* next three things we don't need */
 207   if ( !get_double_and_continue(&buffer, &tmp, TRUE)) return;
 208   if ( !get_double_and_continue(&buffer, &tmp, TRUE)) return;
 209   if ( !get_double_and_continue(&buffer, &tmp, TRUE)) return;
 210
 211
 212   dem->n_columns ++;
 213   (*cur_column) ++;
 214
 215   /* empty spaces for things before that were skipped */
 216   (*cur_row) = (south - dem->min_north) / dem->north_scale;
 217   if ( south > dem->max_north || (*cur_row) < 0 )
 218     (*cur_row) = 0;
 219
 220   n_rows += *cur_row;
 221
 222   g_ptr_array_add ( dem->columns, g_malloc(sizeof(VikDEMColumn)) );
 223   GET_COLUMN(dem,*cur_column)->east_west = east_west;
 224   GET_COLUMN(dem,*cur_column)->south = south;
 225   GET_COLUMN(dem,*cur_column)->n_points = n_rows;
 226   GET_COLUMN(dem,*cur_column)->points = g_malloc(sizeof(gint16)*n_rows);
 227
 228   /* no information for things before that */
 229   for ( i = 0; i < (*cur_row); i++ )
 230     GET_COLUMN(dem,*cur_column)->points[i] = VIK_DEM_INVALID_ELEVATION;
 231
 232   /* now just continue */
 233   dem_parse_block_as_cont ( buffer, dem, cur_column, cur_row );
 234
 235
 236 }
 237
 238 static void dem_parse_block ( gchar *buffer, VikDEM *dem, gint *cur_column, gint *cur_row )
 239 {
 240   /* if haven't read anything or have read all items in a columns and are expecting a new column */
 241   if ( *cur_column == -1 || *cur_row == -1 ) {
 242     dem_parse_block_as_header(buffer, dem, cur_column, cur_row);
 243   } else {
 244     dem_parse_block_as_cont(buffer, dem, cur_column, cur_row);
 245   }
 246 }
 247
 248 /* return size of unzip data or 0 if failed */
 249 /* can be made generic to uncompress zip, gzip, bzip2 data */
 250 static guint uncompress_data(void *uncompressed_buffer, guint uncompressed_size, void *compressed_data, guint compressed_size)
 251 {
 252         z_stream stream;
 253         int err;
 254
 255         stream.next_in = compressed_data;
 256         stream.avail_in = compressed_size;
 257         stream.next_out = uncompressed_buffer;
 258         stream.avail_out = uncompressed_size;
 259         stream.zalloc = (alloc_func)0;
 260         stream.zfree = (free_func)0;
 261         stream.opaque = (voidpf)0;
 262
 263         /* negative windowBits to inflateInit2 means "no header" */
 264         if ((err = inflateInit2(&stream, -MAX_WBITS)) != Z_OK) {
 265                 g_warning("%s(): inflateInit2 failed", __PRETTY_FUNCTION__);
 266                 return 0;
 267         }
 268
 269         err = inflate(&stream, Z_FINISH);
 270         if ((err != Z_OK) && (err != Z_STREAM_END) && stream.msg) {
 271                 g_warning("%s() inflate failed err=%d \"%s\"", __PRETTY_FUNCTION__, err, stream.msg == NULL ? "unknown" : stream.msg);
 272                 inflateEnd(&stream);
 273                 return 0;
 274         }
 275
 276         inflateEnd(&stream);
 277         return(stream.total_out);
 278 }
 279
 280 static void *unzip_hgt_file(gchar *zip_file, gulong *unzip_size)
 281 {
 282         void *unzip_data = NULL;
 283         gchar *zip_data;
 284         struct _lfh {
 285                 guint32 sig;
 286                 guint16 extract_version;
 287                 guint16 flags;
 288                 guint16 comp_method;
 289                 guint16 time;
 290                 guint16 date;
 291                 guint32 crc_32;
 292                 guint32 compressed_size;
 293                 guint32 uncompressed_size;
 294                 guint16 filename_len;
 295                 guint16 extra_field_len;
 296         }  __attribute__ ((__packed__)) *local_file_header = NULL;
 297
 298
 299         local_file_header = (struct _lfh *) zip_file;
 300         if (local_file_header->sig != 0x04034b50) {
 301                 g_warning("%s(): wrong format", __PRETTY_FUNCTION__);
 302                 g_free(unzip_data);
 303                 goto end;
 304         }
 305
 306         zip_data = zip_file + sizeof(struct _lfh) + local_file_header->filename_len + local_file_header->extra_field_len;
 307         unzip_data = g_malloc(local_file_header->uncompressed_size);
 308         gulong uncompressed_size = local_file_header->uncompressed_size;
 309
 310         if (!(*unzip_size = uncompress_data(unzip_data, uncompressed_size, zip_data, local_file_header->compressed_size))) {
 311                 g_free(unzip_data);
 312                 unzip_data = NULL;
 313                 goto end;
 314         }
 315
 316 end:
 317         return(unzip_data);
 318 }
 319
 320 static VikDEM *vik_dem_read_srtm_hgt(const gchar *file_name, const gchar *basename, gboolean zip)
 321 {
 322   gint i, j;
 323   VikDEM *dem;
 324   off_t file_size;
 325   gint16 *dem_mem = NULL;
 326   gchar *dem_file = NULL;
 327   const gint num_rows_3sec = 1201;
 328   const gint num_rows_1sec = 3601;
 329   gint num_rows;
 330   GMappedFile *mf;
 331   gint arcsec;
 332   GError *error = NULL;
 333
 334   dem = g_malloc(sizeof(VikDEM));
 335
 336   dem->horiz_units = VIK_DEM_HORIZ_LL_ARCSECONDS;
 337   dem->orig_vert_units = VIK_DEM_VERT_DECIMETERS;
 338
 339   /* TODO */
 340   dem->min_north = atoi(basename+1) * 3600;
 341   dem->min_east = atoi(basename+4) * 3600;
 342   if ( basename[0] == 'S' )
 343     dem->min_north = - dem->min_north;
 344   if ( basename[3] == 'W' )
 345     dem->min_east = - dem->min_east;
 346
 347   dem->max_north = 3600 + dem->min_north;
 348   dem->max_east = 3600 + dem->min_east;
 349
 350   dem->columns = g_ptr_array_new();
 351   dem->n_columns = 0;
 352
 353   if ((mf = g_mapped_file_new(file_name, FALSE, &error)) == NULL) {
 354     g_error(_("Couldn't map file %s: %s"), file_name, error->message);
 355     g_error_free(error);
 356     g_free(dem);
 357     return NULL;
 358   }
 359   file_size = g_mapped_file_get_length(mf);
 360   dem_file = g_mapped_file_get_contents(mf);
 361
 362   if (zip) {
 363     void *unzip_mem = NULL;
 364     gulong ucsize;
 365
 366     if ((unzip_mem = unzip_hgt_file(dem_file, &ucsize)) == NULL) {
 367       g_mapped_file_free(mf);
 368       g_ptr_array_free(dem->columns, TRUE);
 369       g_free(dem);
 370       return NULL;
 371     }
 372
 373     dem_mem = unzip_mem;
 374     file_size = ucsize;
 375   }
 376
 377   if (file_size == (num_rows_3sec * num_rows_3sec * sizeof(gint16)))
 378     arcsec = 3;
 379   else if (file_size == (num_rows_1sec * num_rows_1sec * sizeof(gint16)))
 380     arcsec = 1;
 381   else {
 382     g_warning("%s(): file %s does not have right size", __PRETTY_FUNCTION__, basename);
 383     g_mapped_file_free(mf);
 384     g_free(dem);
 385     return NULL;
 386   }
 387
 388   num_rows = (arcsec == 3) ? num_rows_3sec : num_rows_1sec;
 389   dem->east_scale = dem->north_scale = arcsec;
 390
 391   for ( i = 0; i < num_rows; i++ ) {
 392     dem->n_columns++;
 393     g_ptr_array_add ( dem->columns, g_malloc(sizeof(VikDEMColumn)) );
 394     GET_COLUMN(dem,i)->east_west = dem->min_east + arcsec*i;
 395     GET_COLUMN(dem,i)->south = dem->min_north;
 396     GET_COLUMN(dem,i)->n_points = num_rows;
 397     GET_COLUMN(dem,i)->points = g_malloc(sizeof(gint16)*num_rows);
 398   }
 399
 400   int ent = 0;
 401   for ( i = (num_rows - 1); i >= 0; i-- ) {
 402     for ( j = 0; j < num_rows; j++ ) {
 403       GET_COLUMN(dem,j)->points[i] = GINT16_FROM_BE(dem_mem[ent]);
 404       ent++;
 405     }
 406
 407   }
 408
 409   if (zip)
 410     g_free(dem_mem);
 411   g_mapped_file_free(mf);
 412   return dem;
 413 }
 414
 415 #define IS_SRTM_HGT(fn) (strlen((fn))==11 && (fn)[7]=='.' && (fn)[8]=='h' && (fn)[9]=='g' && (fn)[10]=='t' && ((fn)[0]=='N' || (fn)[0]=='S') && ((fn)[3]=='E' || (fn)[3]=='W'))
 416
 417 VikDEM *vik_dem_new_from_file(const gchar *file)
 418 {
 419   FILE *f=NULL;
 420   VikDEM *rv;
 421   gchar buffer[DEM_BLOCK_SIZE+1];
 422
 423   /* use to record state for dem_parse_block */
 424   gint cur_column = -1;
 425   gint cur_row = -1;
 426   const gchar *basename = a_file_basename(file);
 427
 428   if ( g_access ( file, R_OK ) != 0 )
 429     return NULL;
 430
 431   if ( (strlen(basename)==11 || ((strlen(basename) == 15) && (basename[11] == '.' && basename[12] == 'z' && basename[13] == 'i' && basename[14] == 'p'))) &&
 432        basename[7]=='.' && basename[8]=='h' && basename[9]=='g' && basename[10]=='t' &&
 433        (basename[0] == 'N' || basename[0] == 'S') && (basename[3] == 'E' || basename[3] =='W')) {
 434     gboolean is_zip_file = (strlen(basename) == 15);
 435     rv = vik_dem_read_srtm_hgt(file, basename, is_zip_file);
 436     return(rv);
 437   }
 438
 439       /* Create Structure */
 440   rv = g_malloc(sizeof(VikDEM));
 441
 442       /* Header */
 443   f = g_fopen(file, "r");
 444   if ( !f ) {
 445     g_free ( rv );
 446     return NULL;
 447   }
 448   buffer[fread(buffer, 1, DEM_BLOCK_SIZE, f)] = '\0';
 449   if ( ! dem_parse_header ( buffer, rv ) ) {
 450     g_free ( rv );
 451     return NULL;
 452   }
 453   /* TODO: actually use header -- i.e. GET # OF COLUMNS EXPECTED */
 454
 455   rv->columns = g_ptr_array_new();
 456   rv->n_columns = 0;
 457
 458       /* Column -- Data */
 459   while (! feof(f) ) {
 460      gchar *tmp;
 461
 462      /* read block */
 463      buffer[fread(buffer, 1, DEM_BLOCK_SIZE, f)] = '\0';
 464
 465      /* Fix Fortran-style exponentiation */
 466      tmp = buffer;
 467      while (*tmp) {
 468        if ( *tmp=='D')
 469          *tmp='E';
 470        tmp++;
 471      }
 472
 473      dem_parse_block(buffer, rv, &cur_column, &cur_row);
 474   }
 475
 476      /* TODO - class C records (right now says 'Invalid' and dies) */
 477
 478   fclose(f);
 479   f = NULL;
 480
 481   /* 24k scale */
 482   if ( rv->horiz_units == VIK_DEM_HORIZ_UTM_METERS && rv->n_columns >= 2 )
 483     rv->north_scale = rv->east_scale = GET_COLUMN(rv, 1)->east_west - GET_COLUMN(rv,0)->east_west;
 484
 485   /* FIXME bug in 10m DEM's */
 486   if ( rv->horiz_units == VIK_DEM_HORIZ_UTM_METERS && rv->north_scale == 10 ) {
 487     rv->min_east -= 100;
 488     rv->min_north += 200;
 489   }
 490
 491
 492   return rv;
 493 }
 494
 495 void vik_dem_free ( VikDEM *dem )
 496 {
 497   guint i;
 498   for ( i = 0; i < dem->n_columns; i++)
 499     g_free ( GET_COLUMN(dem, i)->points );
 500   g_ptr_array_free ( dem->columns, TRUE );
 501   g_free ( dem );
 502 }
 503
 504 gint16 vik_dem_get_xy ( VikDEM *dem, guint col, guint row )
 505 {
 506   if ( col < dem->n_columns )
 507     if ( row < GET_COLUMN(dem, col)->n_points )
 508       return GET_COLUMN(dem, col)->points[row];
 509   return VIK_DEM_INVALID_ELEVATION;
 510 }
 511
 512 gint16 vik_dem_get_east_north ( VikDEM *dem, gdouble east, gdouble north )
 513 {
 514   gint col, row;
 515
 516   if ( east > dem->max_east || east < dem->min_east ||
 517       north > dem->max_north || north < dem->min_north )
 518     return VIK_DEM_INVALID_ELEVATION;
 519
 520   col = (gint) floor((east - dem->min_east) / dem->east_scale);
 521   row = (gint) floor((north - dem->min_north) / dem->north_scale);
 522
 523   return vik_dem_get_xy ( dem, col, row );
 524 }
 525
 526 static gboolean dem_get_ref_points_elev_dist(VikDEM *dem,
 527     gdouble east, gdouble north, /* in seconds */
 528     gint16 *elevs, gint16 *dists)
 529 {
 530   int i;
 531   int cols[4], rows[4];
 532   struct LatLon ll[4];
 533   struct LatLon pos;
 534
 535   if ( east > dem->max_east || east < dem->min_east ||
 536       north > dem->max_north || north < dem->min_north )
 537     return FALSE;  /* got nothing */
 538
 539   pos.lon = east/3600;
 540   pos.lat = north/3600;
 541
 542   /* order of the data: sw, nw, ne, se */
 543   /* sw */
 544   cols[0] = (gint) floor((east - dem->min_east) / dem->east_scale);
 545   rows[0] = (gint) floor((north - dem->min_north) / dem->north_scale);
 546   ll[0].lon = (dem->min_east + dem->east_scale*cols[0])/3600;
 547   ll[0].lat = (dem->min_north + dem->north_scale*rows[0])/3600;
 548   /* nw */
 549   cols[1] = cols[0];
 550   rows[1] = rows[0] + 1;
 551   ll[1].lon = ll[0].lon;
 552   ll[1].lat = ll[0].lat + (gdouble)dem->north_scale/3600;
 553   /* ne */
 554   cols[2] = cols[0] + 1;
 555   rows[2] = rows[0] + 1;
 556   ll[2].lon = ll[0].lon + (gdouble)dem->east_scale/3600;
 557   ll[2].lat = ll[0].lat + (gdouble)dem->north_scale/3600;
 558   /* se */
 559   cols[3] = cols[0] + 1;
 560   rows[3] = rows[0];
 561   ll[3].lon = ll[0].lon + (gdouble)dem->east_scale/3600;
 562   ll[3].lat = ll[0].lat;
 563
 564   for (i = 0; i < 4; i++) {
 565     if ((elevs[i] = vik_dem_get_xy(dem, cols[i], rows[i])) == VIK_DEM_INVALID_ELEVATION)
 566       return FALSE;
 567     dists[i] = a_coords_latlon_diff(&pos, &ll[i]);
 568   }
 569
 570 #if 0  /* debug */
 571   for (i = 0; i < 4; i++)
 572     fprintf(stderr, "%f:%f:%d:%d  ", ll[i].lat, ll[i].lon, dists[i], elevs[i]);
 573   fprintf(stderr, "   north_scale=%f\n", dem->north_scale);
 574 #endif
 575
 576   return TRUE;  /* all OK */
 577 }
 578
 579 gint16 vik_dem_get_simple_interpol ( VikDEM *dem, gdouble east, gdouble north )
 580 {
 581   int i;
 582   gint16 elevs[4], dists[4];
 583
 584   if (!dem_get_ref_points_elev_dist(dem, east, north, elevs, dists))
 585     return VIK_DEM_INVALID_ELEVATION;
 586
 587   for (i = 0; i < 4; i++) {
 588     if (dists[i] < 1) {
 589       return(elevs[i]);
 590     }
 591   }
 592
 593   gdouble t = (gdouble)elevs[0]/dists[0] + (gdouble)elevs[1]/dists[1] + (gdouble)elevs[2]/dists[2] + (gdouble)elevs[3]/dists[3];
 594   gdouble b = 1.0/dists[0] + 1.0/dists[1] + 1.0/dists[2] + 1.0/dists[3];
 595
 596   return(t/b);
 597 }
 598
 599 gint16 vik_dem_get_shepard_interpol ( VikDEM *dem, gdouble east, gdouble north )
 600 {
 601   int i;
 602   gint16 elevs[4], dists[4];
 603   gint16 max_dist;
 604   gdouble t = 0.0;
 605   gdouble b = 0.0;
 606
 607   if (!dem_get_ref_points_elev_dist(dem, east, north, elevs, dists))
 608     return VIK_DEM_INVALID_ELEVATION;
 609
 610   max_dist = 0;
 611   for (i = 0; i < 4; i++) {
 612     if (dists[i] < 1) {
 613       return(elevs[i]);
 614     }
 615     if (dists[i] > max_dist)
 616       max_dist = dists[i];
 617   }
 618
 619   gdouble tmp;
 620 #if 0 /* derived method by Franke & Nielson. Does not seem to work too well here */
 621   for (i = 0; i < 4; i++) {
 622     tmp = pow((1.0*(max_dist - dists[i])/max_dist*dists[i]), 2);
 623     t += tmp*elevs[i];
 624     b += tmp;
 625   }
 626 #endif
 627
 628   for (i = 0; i < 4; i++) {
 629     tmp = pow((1.0/dists[i]), 2);
 630     t += tmp*elevs[i];
 631     b += tmp;
 632   }
 633
 634   // fprintf(stderr, "DEBUG: tmp=%f t=%f b=%f %f\n", tmp, t, b, t/b);
 635
 636   return(t/b);
 637
 638 }
 639
 640 void vik_dem_east_north_to_xy ( VikDEM *dem, gdouble east, gdouble north, guint *col, guint *row )
 641 {
 642   *col = (gint) floor((east - dem->min_east) / dem->east_scale);
 643   *row = (gint) floor((north - dem->min_north) / dem->north_scale);
 644   if ( *col < 0 ) *col = 0;
 645   if ( *row < 0 ) *row = 0;
 646 }
 647