src/Environment/environment.cxx

   1 // environment.cxx -- routines to model the natural environment
   2 //
   3 // Written by David Megginson, started February 2002.
   4 //
   5 // Copyright (C) 2002  David Megginson - david@megginson.com
   6 //
   7 // This program is free software; you can redistribute it and/or
   8 // modify it under the terms of the GNU General Public License as
   9 // published by the Free Software Foundation; either version 2 of the
  10 // License, or (at your option) any later version.
  11 //
  12 // This program is distributed in the hope that it will be useful, but
  13 // WITHOUT ANY WARRANTY; without even the implied warranty of
  14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15 // General Public License for more details.
  16 //
  17 // You should have received a copy of the GNU General Public License
  18 // along with this program; if not, write to the Free Software
  19 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  20 //
  21 // $Id$
  22
  23
  24 #ifdef HAVE_CONFIG_H
  25 #  include <config.h>
  26 #endif
  27
  28 #include <math.h>
  29
  30 #include <boost/tuple/tuple.hpp>
  31
  32 #include <simgear/constants.h>
  33 #include <simgear/debug/logstream.hxx>
  34 #include <simgear/math/interpolater.hxx>
  35 #include <simgear/props/props.hxx>
  36 #include <simgear/environment/visual_enviro.hxx>
  37
  38 #include <Main/fg_props.hxx>
  39 #include <signal.h>
  40
  41 #include "environment.hxx"
  42 #include "atmosphere.hxx"
  43
  44 \f
  45 ////////////////////////////////////////////////////////////////////////
  46 // Atmosphere model.
  47 ////////////////////////////////////////////////////////////////////////
  48
  49 #ifdef USING_TABLES
  50
  51 // Calculated based on the ISA standard day, as found at e.g.
  52 // http://www.av8n.com/physics/altimetry.htm
  53
  54 // Each line of data has 3 elements:
  55 //   Elevation (ft),
  56 //   temperature factor (dimensionless ratio of absolute temp),
  57 //   pressure factor (dimensionless ratio)
  58 static double atmosphere_data[][3] = {
  59  {  -3000.00,   1.021,  1.1133 },
  60  {      0.00,   1.000,  1.0000 },
  61  {   2952.76,   0.980,  0.8978 },
  62  {   5905.51,   0.959,  0.8042 },
  63  {   8858.27,   0.939,  0.7187 },
  64  {  11811.02,   0.919,  0.6407 },
  65  {  14763.78,   0.898,  0.5697 },
  66  {  17716.54,   0.878,  0.5052 },
  67  {  20669.29,   0.858,  0.4468 },
  68  {  23622.05,   0.838,  0.3940 },
  69  {  26574.80,   0.817,  0.3463 },
  70  {  29527.56,   0.797,  0.3034 },
  71  {  32480.31,   0.777,  0.2649 },
  72  {  35433.07,   0.756,  0.2305 },
  73  {  38385.83,   0.752,  0.2000 },
  74  {  41338.58,   0.752,  0.1736 },
  75  {  44291.34,   0.752,  0.1506 },
  76  {  47244.09,   0.752,  0.1307 },
  77  {  50196.85,   0.752,  0.1134 },
  78  {  53149.61,   0.752,  0.0984 },
  79  {  56102.36,   0.752,  0.0854 },
  80  {  59055.12,   0.752,  0.0741 },
  81  {  62007.87,   0.752,  0.0643 },
  82  {  65000.00,   0.752,  0.0557 },
  83  {  68000.00,   0.754,  0.0482 },
  84  {  71000.00,   0.758,  0.0418 },
  85  {  74000.00,   0.761,  0.0362 },
  86  {  77000.00,   0.764,  0.0314 },
  87  {  80000.00,   0.767,  0.0273 },
  88  {  83000.00,   0.770,  0.0237 },
  89  {  86000.00,   0.773,  0.0206 },
  90  {  89000.00,   0.777,  0.0179 },
  91  {  92000.00,   0.780,  0.0156 },
  92  {  95000.00,   0.783,  0.0135 },
  93  {  98000.00,   0.786,  0.0118 },
  94  { 101000.00,   0.789,  0.0103 },
  95  { -1, -1, -1 }
  96 };
  97
  98 static SGInterpTable * _temperature_degc_table = 0;
  99 static SGInterpTable * _pressure_inhg_table = 0;
 100
 101 static void
 102 _setup_tables ()
 103 {
 104   if (_temperature_degc_table != 0)
 105       return;
 106
 107   _temperature_degc_table = new SGInterpTable;
 108   _pressure_inhg_table = new SGInterpTable;
 109
 110   for (int i = 0; atmosphere_data[i][0] != -1; i++) {
 111     _temperature_degc_table->addEntry(atmosphere_data[i][0],
 112                                       atmosphere_data[i][1]);
 113     _pressure_inhg_table->addEntry(atmosphere_data[i][0],
 114                                    atmosphere_data[i][2]);
 115   }
 116 }
 117 #endif
 118
 119 \f
 120 ////////////////////////////////////////////////////////////////////////
 121 // Implementation of FGEnvironment.
 122 ////////////////////////////////////////////////////////////////////////
 123
 124 void FGEnvironment::_init()
 125 {
 126     elevation_ft = 0;
 127     visibility_m = 32000;
 128     temperature_sea_level_degc = 15;
 129     temperature_degc = 15;
 130     dewpoint_sea_level_degc = 5; // guess
 131     dewpoint_degc = 5;
 132     pressure_sea_level_inhg = 29.92;
 133     pressure_inhg = 29.92;
 134     turbulence_magnitude_norm = 0;
 135     turbulence_rate_hz = 1;
 136     wind_from_heading_deg = 0;
 137     wind_speed_kt = 0;
 138     wind_from_north_fps = 0;
 139     wind_from_east_fps = 0;
 140     wind_from_down_fps = 0;
 141     thermal_lift_fps = 0;
 142     ridge_lift_fps= 0;
 143     local_weather_lift_fps=0;
 144     altitude_half_to_sun_m = 1000;
 145     altitude_tropo_top_m = 10000;
 146 #ifdef USING_TABLES
 147     _setup_tables();
 148 #endif
 149     _recalc_density();
 150     _recalc_relative_humidity();
 151     live_update = true;
 152 }
 153
 154 FGEnvironment::FGEnvironment()
 155 {
 156     _init();
 157 }
 158
 159 FGEnvironment::FGEnvironment (const FGEnvironment &env)
 160 {
 161     _init();
 162     copy(env);
 163 }
 164
 165 FGEnvironment::~FGEnvironment()
 166 {
 167 }
 168
 169 void
 170 FGEnvironment::copy (const FGEnvironment &env)
 171 {
 172     elevation_ft = env.elevation_ft;
 173     visibility_m = env.visibility_m;
 174     temperature_sea_level_degc = env.temperature_sea_level_degc;
 175     temperature_degc = env.temperature_degc;
 176     dewpoint_sea_level_degc = env.dewpoint_sea_level_degc;
 177     dewpoint_degc = env.dewpoint_degc;
 178     pressure_sea_level_inhg = env.pressure_sea_level_inhg;
 179     wind_from_heading_deg = env.wind_from_heading_deg;
 180     wind_speed_kt = env.wind_speed_kt;
 181     wind_from_north_fps = env.wind_from_north_fps;
 182     wind_from_east_fps = env.wind_from_east_fps;
 183     wind_from_down_fps = env.wind_from_down_fps;
 184     thermal_lift_fps = env.thermal_lift_fps;
 185     ridge_lift_fps= env.ridge_lift_fps;
 186     local_weather_lift_fps = env.local_weather_lift_fps;
 187     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 188     turbulence_rate_hz = env.turbulence_rate_hz;
 189 }
 190
 191 static inline bool
 192 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 193                   const char * name, void (FGEnvironment::*setter)(double))
 194 {
 195     const SGPropertyNode * child = node->getNode(name);
 196                                 // fragile: depends on not being typed
 197                                 // as a number
 198     if (child != 0 && child->hasValue() &&
 199         child->getStringValue()[0] != '\0') {
 200         (env->*setter)(child->getDoubleValue());
 201         return true;
 202     } else {
 203         return false;
 204     }
 205 }
 206
 207 void
 208 FGEnvironment::read (const SGPropertyNode * node)
 209 {
 210     bool live_update = set_live_update( false );
 211     maybe_copy_value(this, node, "visibility-m",
 212                      &FGEnvironment::set_visibility_m);
 213
 214     maybe_copy_value(this, node, "elevation-ft",
 215                      &FGEnvironment::set_elevation_ft);
 216
 217     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 218                           &FGEnvironment::set_temperature_sea_level_degc)) {
 219         if( maybe_copy_value(this, node, "temperature-degc",
 220                          &FGEnvironment::set_temperature_degc)) {
 221             _recalc_sl_temperature();
 222         }
 223     }
 224
 225     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 226                           &FGEnvironment::set_dewpoint_sea_level_degc)) {
 227         if( maybe_copy_value(this, node, "dewpoint-degc",
 228                          &FGEnvironment::set_dewpoint_degc)) {
 229             _recalc_sl_dewpoint();
 230         }
 231     }
 232
 233     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 234                           &FGEnvironment::set_pressure_sea_level_inhg)) {
 235         if( maybe_copy_value(this, node, "pressure-inhg",
 236                          &FGEnvironment::set_pressure_inhg)) {
 237             _recalc_sl_pressure();
 238         }
 239    }
 240
 241     maybe_copy_value(this, node, "wind-from-heading-deg",
 242                      &FGEnvironment::set_wind_from_heading_deg);
 243
 244     maybe_copy_value(this, node, "wind-speed-kt",
 245                      &FGEnvironment::set_wind_speed_kt);
 246
 247     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 248                      &FGEnvironment::set_turbulence_magnitude_norm);
 249
 250     maybe_copy_value(this, node, "turbulence/rate-hz",
 251                      &FGEnvironment::set_turbulence_rate_hz);
 252
 253     // calculate derived properties here to avoid duplicate expensive computations
 254     _recalc_ne();
 255     _recalc_alt_pt();
 256     _recalc_alt_dewpoint();
 257     _recalc_density();
 258     _recalc_relative_humidity();
 259
 260     set_live_update(live_update);
 261 }
 262
 263
 264 double
 265 FGEnvironment::get_visibility_m () const
 266 {
 267   return visibility_m;
 268 }
 269
 270 double
 271 FGEnvironment::get_temperature_sea_level_degc () const
 272 {
 273   return temperature_sea_level_degc;
 274 }
 275
 276 double
 277 FGEnvironment::get_temperature_degc () const
 278 {
 279   return temperature_degc;
 280 }
 281
 282 double
 283 FGEnvironment::get_temperature_degf () const
 284 {
 285   return (temperature_degc * 9.0 / 5.0) + 32.0;
 286 }
 287
 288 double
 289 FGEnvironment::get_dewpoint_sea_level_degc () const
 290 {
 291   return dewpoint_sea_level_degc;
 292 }
 293
 294 double
 295 FGEnvironment::get_dewpoint_degc () const
 296 {
 297   return dewpoint_degc;
 298 }
 299
 300 double
 301 FGEnvironment::get_pressure_sea_level_inhg () const
 302 {
 303   return pressure_sea_level_inhg;
 304 }
 305
 306 double
 307 FGEnvironment::get_pressure_inhg () const
 308 {
 309   return pressure_inhg;
 310 }
 311
 312 double
 313 FGEnvironment::get_density_slugft3 () const
 314 {
 315   return density_slugft3;
 316 }
 317
 318 double
 319 FGEnvironment::get_relative_humidity () const
 320 {
 321   return relative_humidity;
 322 }
 323
 324 double
 325 FGEnvironment::get_density_tropo_avg_kgm3 () const
 326 {
 327   return density_tropo_avg_kgm3;
 328 }
 329
 330 double
 331 FGEnvironment::get_altitude_half_to_sun_m () const
 332 {
 333   return altitude_half_to_sun_m;
 334 }
 335
 336 double
 337 FGEnvironment::get_altitude_tropo_top_m () const
 338 {
 339   return altitude_tropo_top_m;
 340 }
 341
 342 double
 343 FGEnvironment::get_wind_from_heading_deg () const
 344 {
 345   return wind_from_heading_deg;
 346 }
 347
 348 double
 349 FGEnvironment::get_wind_speed_kt () const
 350 {
 351   return wind_speed_kt;
 352 }
 353
 354 double
 355 FGEnvironment::get_wind_from_north_fps () const
 356 {
 357   return wind_from_north_fps;
 358 }
 359
 360 double
 361 FGEnvironment::get_wind_from_east_fps () const
 362 {
 363   return wind_from_east_fps;
 364 }
 365
 366 double
 367 FGEnvironment::get_wind_from_down_fps () const
 368 {
 369   return wind_from_down_fps;
 370 }
 371
 372 double
 373 FGEnvironment::get_thermal_lift_fps () const
 374 {
 375   return thermal_lift_fps;
 376 }
 377
 378 double
 379 FGEnvironment::get_ridge_lift_fps () const
 380 {
 381   return ridge_lift_fps;
 382 }
 383
 384 double
 385 FGEnvironment::get_local_weather_lift_fps () const
 386 {
 387   return local_weather_lift_fps;
 388 }
 389
 390 double
 391 FGEnvironment::get_turbulence_magnitude_norm () const
 392 {
 393   if( sgEnviro.get_turbulence_enable_state() )
 394     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 395       return sgEnviro.get_cloud_turbulence();
 396   return turbulence_magnitude_norm;
 397 }
 398
 399 double
 400 FGEnvironment::get_turbulence_rate_hz () const
 401 {
 402   return turbulence_rate_hz;
 403 }
 404
 405 double
 406 FGEnvironment::get_elevation_ft () const
 407 {
 408   return elevation_ft;
 409 }
 410
 411 void
 412 FGEnvironment::set_visibility_m (double v)
 413 {
 414   visibility_m = v;
 415 }
 416
 417 void
 418 FGEnvironment::set_temperature_sea_level_degc (double t)
 419 {
 420   temperature_sea_level_degc = t;
 421   if (dewpoint_sea_level_degc > t)
 422       dewpoint_sea_level_degc = t;
 423   if( live_update ) {
 424     _recalc_alt_pt();
 425     _recalc_density();
 426   }
 427 }
 428
 429 void
 430 FGEnvironment::set_temperature_degc (double t)
 431 {
 432   temperature_degc = t;
 433   if( live_update ) {
 434     _recalc_sl_temperature();
 435     _recalc_sl_pressure();
 436     _recalc_alt_pt();
 437     _recalc_density();
 438     _recalc_relative_humidity();
 439   }
 440 }
 441
 442 void
 443 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 444 {
 445   dewpoint_sea_level_degc = t;
 446   if (temperature_sea_level_degc < t)
 447       temperature_sea_level_degc = t;
 448   if( live_update ) {
 449     _recalc_alt_dewpoint();
 450     _recalc_density();
 451   }
 452 }
 453
 454 void
 455 FGEnvironment::set_dewpoint_degc (double t)
 456 {
 457   dewpoint_degc = t;
 458   if( live_update ) {
 459     _recalc_sl_dewpoint();
 460     _recalc_density();
 461     _recalc_relative_humidity();
 462   }
 463 }
 464
 465 void
 466 FGEnvironment::set_pressure_sea_level_inhg (double p)
 467 {
 468   pressure_sea_level_inhg = p;
 469   if( live_update ) {
 470     _recalc_alt_pt();
 471     _recalc_density();
 472   }
 473 }
 474
 475 void
 476 FGEnvironment::set_pressure_inhg (double p)
 477 {
 478   pressure_inhg = p;
 479   if( live_update ) {
 480     _recalc_sl_pressure();
 481     _recalc_density();
 482   }
 483 }
 484
 485 void
 486 FGEnvironment::set_wind_from_heading_deg (double h)
 487 {
 488   wind_from_heading_deg = h;
 489   if( live_update ) {
 490     _recalc_ne();
 491   }
 492 }
 493
 494 void
 495 FGEnvironment::set_wind_speed_kt (double s)
 496 {
 497   wind_speed_kt = s;
 498   if( live_update ) {
 499     _recalc_ne();
 500   }
 501 }
 502
 503 void
 504 FGEnvironment::set_wind_from_north_fps (double n)
 505 {
 506   wind_from_north_fps = n;
 507   if( live_update ) {
 508     _recalc_hdgspd();
 509   }
 510 }
 511
 512 void
 513 FGEnvironment::set_wind_from_east_fps (double e)
 514 {
 515   wind_from_east_fps = e;
 516   if( live_update ) {
 517     _recalc_hdgspd();
 518   }
 519 }
 520
 521 void
 522 FGEnvironment::set_wind_from_down_fps (double d)
 523 {
 524   wind_from_down_fps = d;
 525   if( live_update ) {
 526     _recalc_hdgspd();
 527   }
 528 }
 529
 530 void
 531 FGEnvironment::set_thermal_lift_fps (double th)
 532 {
 533   thermal_lift_fps = th;
 534   if( live_update ) {
 535     _recalc_updraft();
 536   }
 537 }
 538
 539 void
 540 FGEnvironment::set_ridge_lift_fps (double ri)
 541 {
 542   ridge_lift_fps = ri;
 543   if( live_update ) {
 544     _recalc_updraft();
 545   }
 546 }
 547
 548 void
 549 FGEnvironment::set_local_weather_lift_fps (double lwl)
 550 {
 551   local_weather_lift_fps = lwl;
 552   if( live_update ) {
 553     _recalc_updraft();
 554   }
 555 }
 556
 557 void
 558 FGEnvironment::set_turbulence_magnitude_norm (double t)
 559 {
 560   turbulence_magnitude_norm = t;
 561 }
 562
 563 void
 564 FGEnvironment::set_turbulence_rate_hz (double r)
 565 {
 566   turbulence_rate_hz = r;
 567 }
 568
 569 void
 570 FGEnvironment::set_elevation_ft (double e)
 571 {
 572   elevation_ft = e;
 573   if( live_update ) {
 574     _recalc_alt_pt();
 575     _recalc_alt_dewpoint();
 576     _recalc_density();
 577     _recalc_relative_humidity();
 578   }
 579 }
 580
 581 void
 582 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 583 {
 584   altitude_half_to_sun_m = alt;
 585   if( live_update ) {
 586     _recalc_density_tropo_avg_kgm3();
 587   }
 588 }
 589
 590 void
 591 FGEnvironment::set_altitude_tropo_top_m (double alt)
 592 {
 593   altitude_tropo_top_m = alt;
 594   if( live_update ) {
 595     _recalc_density_tropo_avg_kgm3();
 596   }
 597 }
 598
 599
 600 void
 601 FGEnvironment::_recalc_hdgspd ()
 602 {
 603   double angle_rad;
 604
 605   if (wind_from_east_fps == 0) {
 606     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 607   } else {
 608     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 609   }
 610   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 611   if (wind_from_east_fps >= 0)
 612     wind_from_heading_deg = 90 - wind_from_heading_deg;
 613   else
 614     wind_from_heading_deg = 270 - wind_from_heading_deg;
 615
 616 #if 0
 617   // FIXME: Windspeed can become negative with these formulas.
 618   //        which can cause problems for animations that rely
 619   //        on the windspeed property.
 620   if (angle_rad == 0)
 621     wind_speed_kt = fabs(wind_from_east_fps
 622                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 623   else
 624     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 625       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 626 #else
 627   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 628                        wind_from_east_fps * wind_from_east_fps)
 629                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 630 #endif
 631 }
 632
 633 void
 634 FGEnvironment::_recalc_ne ()
 635 {
 636   double speed_fps =
 637     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 638
 639   wind_from_north_fps = speed_fps *
 640     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 641   wind_from_east_fps = speed_fps *
 642     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 643 }
 644
 645 void
 646 FGEnvironment::_recalc_updraft ()
 647 {
 648   wind_from_down_fps = thermal_lift_fps + ridge_lift_fps + local_weather_lift_fps ;
 649 }
 650
 651 // Intended to help with the interpretation of METAR data,
 652 // not for random in-flight outside-air temperatures.
 653 void
 654 FGEnvironment::_recalc_sl_temperature ()
 655 {
 656
 657 #if 0
 658   {
 659     SG_LOG(SG_GENERAL, SG_DEBUG, "recalc_sl_temperature: using "
 660       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 661   }
 662 #endif
 663
 664   if (elevation_ft * atmodel::foot >= ISA_def[1].height) {
 665     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_temperature: "
 666         << "valid only in troposphere, not " << elevation_ft);
 667     return;
 668   }
 669
 670 // Clamp: temperature of the stratosphere, in degrees C:
 671   double t_strato = ISA_def[1].temp - atmodel::freezing;
 672   if (temperature_degc < t_strato) temperature_sea_level_degc = t_strato;
 673   else temperature_sea_level_degc =
 674       temperature_degc + elevation_ft * atmodel::foot * ISA_def[0].lapse;
 675
 676 // Alternative implemenation:
 677 //  else temperature_sea_level_inhg = T_layer(0., elevation_ft * foot,
 678 //      pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) - freezing;
 679 }
 680
 681 void
 682 FGEnvironment::_recalc_sl_dewpoint ()
 683 {
 684                                 // 0.2degC/1000ft
 685                                 // FIXME: this will work only for low
 686                                 // elevations
 687   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 688   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 689     dewpoint_sea_level_degc = temperature_sea_level_degc;
 690 }
 691
 692 void
 693 FGEnvironment::_recalc_alt_dewpoint ()
 694 {
 695                                 // 0.2degC/1000ft
 696                                 // FIXME: this will work only for low
 697                                 // elevations
 698   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 699   if (dewpoint_degc > temperature_degc)
 700     dewpoint_degc = temperature_degc;
 701 }
 702
 703 void
 704 FGEnvironment::_recalc_sl_pressure ()
 705 {
 706   using namespace atmodel;
 707 #if 0
 708   {
 709     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_pressure: using "
 710       << pressure_inhg << " and "
 711       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 712   }
 713 #endif
 714   pressure_sea_level_inhg = P_layer(0., elevation_ft * foot,
 715       pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) / inHg;
 716 }
 717
 718 // This gets called at frame rate, to account for the aircraft's
 719 // changing altitude.
 720 // Called by set_elevation_ft() which is called by FGEnvironmentMgr::update
 721
 722 void
 723 FGEnvironment::_recalc_alt_pt ()
 724 {
 725   using namespace atmodel;
 726 #if 0
 727   {
 728     static int count(0);
 729     if (++count % 1000 == 0) {
 730       SG_LOG(SG_GENERAL, SG_ALERT,
 731            "recalc_alt_pt for: " << elevation_ft
 732         << "  using "  << pressure_sea_level_inhg
 733         << "  and "  << temperature_sea_level_degc
 734         << " :: " << this
 735         << "  # " << count);
 736     }
 737   }
 738 #endif
 739   double press, temp;
 740   boost::tie(press, temp) = PT_vs_hpt(elevation_ft * foot,
 741         pressure_sea_level_inhg * inHg, temperature_sea_level_degc + freezing);
 742   temperature_degc = temp - freezing;
 743   pressure_inhg = press / inHg;
 744 }
 745
 746 void
 747 FGEnvironment::_recalc_density ()
 748 {
 749   double pressure_psf = pressure_inhg * 70.7487;
 750
 751   // adjust for humidity
 752   // calculations taken from USA Today (oops!) at
 753   // http://www.usatoday.com/weather/basics/density-calculations.htm
 754   double temperature_degk = temperature_degc + 273.15;
 755   double pressure_mb = pressure_inhg * 33.86;
 756   double vapor_pressure_mb =
 757     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 758   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 759   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 760
 761   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 762   _recalc_density_tropo_avg_kgm3();
 763 }
 764
 765 // This is used to calculate the average density on the path
 766 // of sunlight to the observer for calculating sun-color
 767 void
 768 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 769 {
 770   double pressure_mb = pressure_inhg * 33.86;
 771   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 772
 773   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 774
 775   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 776       / (287.05 * virtual_temp);
 777   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 778       / ( 287.05 * virtual_temp);
 779
 780   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 781 }
 782
 783 void
 784 FGEnvironment::_recalc_relative_humidity ()
 785 {
 786 /*
 787   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 788   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 789       / ( 237.7 + temperature_degc)) );
 790   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 791
 792   with a little algebra, this gets the same result and spares two multiplications and one pow()
 793 */
 794   double a = (7.5 * dewpoint_degc)    / ( 237.7 + dewpoint_degc);
 795   double b = (7.5 * temperature_degc) / ( 237.7 + temperature_degc);
 796   relative_humidity = 100 * pow(10.0,a-b);
 797 }
 798
 799 bool
 800 FGEnvironment::set_live_update( bool _live_update )
 801 {
 802   bool b = live_update;
 803   live_update = _live_update;
 804   return b;
 805 }
 806
 807
 808 ////////////////////////////////////////////////////////////////////////
 809 // Functions.
 810 ////////////////////////////////////////////////////////////////////////
 811
 812 static inline double
 813 do_interp (double a, double b, double fraction)
 814 {
 815     double retval = (a + ((b - a) * fraction));
 816     return retval;
 817 }
 818
 819 static inline double
 820 do_interp_deg (double a, double b, double fraction)
 821 {
 822     a = fmod(a, 360);
 823     b = fmod(b, 360);
 824     if (fabs(b-a) > 180) {
 825         if (a < b)
 826             a += 360;
 827         else
 828             b += 360;
 829     }
 830     return fmod(do_interp(a, b, fraction), 360);
 831 }
 832
 833 void
 834 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 835              double fraction, FGEnvironment * result)
 836 {
 837     // don't calculate each internal property every time we set a single value
 838     // we trigger that at the end of the interpolation process
 839     bool live_update = result->set_live_update( false );
 840
 841     result->set_visibility_m
 842         (do_interp(env1->get_visibility_m(),
 843                    env2->get_visibility_m(),
 844                    fraction));
 845
 846     result->set_temperature_sea_level_degc
 847         (do_interp(env1->get_temperature_sea_level_degc(),
 848                    env2->get_temperature_sea_level_degc(),
 849                    fraction));
 850
 851     result->set_dewpoint_sea_level_degc
 852         (do_interp(env1->get_dewpoint_sea_level_degc(),
 853                    env2->get_dewpoint_sea_level_degc(),
 854                    fraction));
 855
 856     result->set_pressure_sea_level_inhg
 857         (do_interp(env1->get_pressure_sea_level_inhg(),
 858                    env2->get_pressure_sea_level_inhg(),
 859                    fraction));
 860
 861     result->set_wind_from_heading_deg
 862         (do_interp_deg(env1->get_wind_from_heading_deg(),
 863                        env2->get_wind_from_heading_deg(),
 864                        fraction));
 865
 866     result->set_wind_speed_kt
 867         (do_interp(env1->get_wind_speed_kt(),
 868                    env2->get_wind_speed_kt(),
 869                    fraction));
 870
 871     result->set_elevation_ft
 872         (do_interp(env1->get_elevation_ft(),
 873                    env2->get_elevation_ft(),
 874                    fraction));
 875
 876     result->set_turbulence_magnitude_norm
 877         (do_interp(env1->get_turbulence_magnitude_norm(),
 878                    env2->get_turbulence_magnitude_norm(),
 879                    fraction));
 880
 881     result->set_turbulence_rate_hz
 882         (do_interp(env1->get_turbulence_rate_hz(),
 883                    env2->get_turbulence_rate_hz(),
 884                    fraction));
 885
 886     // calculate derived properties here to avoid duplicate expensive computations
 887     result->_recalc_ne();
 888     result->_recalc_alt_pt();
 889     result->_recalc_alt_dewpoint();
 890     result->_recalc_density();
 891     result->_recalc_relative_humidity();
 892
 893     result->set_live_update(live_update);
 894 }
 895
 896 // end of environment.cxx