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author | Lorenz Meier <lm@inf.ethz.ch> | 2014-07-07 00:05:29 +0200 |
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committer | Lorenz Meier <lm@inf.ethz.ch> | 2014-07-07 00:05:29 +0200 |
commit | 5559e568b6e5502ad51ec96fd531876c1191d641 (patch) | |
tree | bc6356b5bed0481cced4a7d7e7ef99bd225d91a7 /src/lib/geo_lookup/geo_mag_declination.c | |
parent | 863385dbc4243c8ecb19890a0dfce4a0b7ead9d6 (diff) | |
parent | d67089b23f58ac152253f58c5deaebbd57db0362 (diff) | |
download | px4-firmware-safelink.tar.gz px4-firmware-safelink.tar.bz2 px4-firmware-safelink.zip |
Merged master into safelinksafelink
Diffstat (limited to 'src/lib/geo_lookup/geo_mag_declination.c')
-rw-r--r-- | src/lib/geo_lookup/geo_mag_declination.c | 131 |
1 files changed, 131 insertions, 0 deletions
diff --git a/src/lib/geo_lookup/geo_mag_declination.c b/src/lib/geo_lookup/geo_mag_declination.c new file mode 100644 index 000000000..c41d52606 --- /dev/null +++ b/src/lib/geo_lookup/geo_mag_declination.c @@ -0,0 +1,131 @@ +/**************************************************************************** + * + * Copyright (c) 2014 MAV GEO Library (MAVGEO). All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions + * are met: + * + * 1. Redistributions of source code must retain the above copyright + * notice, this list of conditions and the following disclaimer. + * 2. Redistributions in binary form must reproduce the above copyright + * notice, this list of conditions and the following disclaimer in + * the documentation and/or other materials provided with the + * distribution. + * 3. Neither the name MAVGEO nor the names of its contributors may be + * used to endorse or promote products derived from this software + * without specific prior written permission. + * + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS + * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT + * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS + * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE + * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, + * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, + * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS + * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED + * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN + * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE + * POSSIBILITY OF SUCH DAMAGE. + * + ****************************************************************************/ + +/** +* @file geo_mag_declination.c +* +* Calculation / lookup table for earth magnetic field declination. +* +* Lookup table from Scott Ferguson <scottfromscott@gmail.com> +* +* XXX Lookup table currently too coarse in resolution (only full degrees) +* and lat/lon res - needs extension medium term. +* +*/ + +#include <geo/geo.h> + +/** set this always to the sampling in degrees for the table below */ +#define SAMPLING_RES 10.0f +#define SAMPLING_MIN_LAT -60.0f +#define SAMPLING_MAX_LAT 60.0f +#define SAMPLING_MIN_LON -180.0f +#define SAMPLING_MAX_LON 180.0f + +static const int8_t declination_table[13][37] = \ +{ + { 46, 45, 44, 42, 41, 40, 38, 36, 33, 28, 23, 16, 10, 4, -1, -5, -9, -14, -19, -26, -33, -40, -48, -55, -61, -66, -71, -74, -75, -72, -61, -25, 22, 40, 45, 47, 46 }, + { 30, 30, 30, 30, 29, 29, 29, 29, 27, 24, 18, 11, 3, -3, -9, -12, -15, -17, -21, -26, -32, -39, -45, -51, -55, -57, -56, -53, -44, -31, -14, 0, 13, 21, 26, 29, 30 }, + { 21, 22, 22, 22, 22, 22, 22, 22, 21, 18, 13, 5, -3, -11, -17, -20, -21, -22, -23, -25, -29, -35, -40, -44, -45, -44, -40, -32, -22, -12, -3, 3, 9, 14, 18, 20, 21 }, + { 16, 17, 17, 17, 17, 17, 16, 16, 16, 13, 8, 0, -9, -16, -21, -24, -25, -25, -23, -20, -21, -24, -28, -31, -31, -29, -24, -17, -9, -3, 0, 4, 7, 10, 13, 15, 16 }, + { 12, 13, 13, 13, 13, 13, 12, 12, 11, 9, 3, -4, -12, -19, -23, -24, -24, -22, -17, -12, -9, -10, -13, -17, -18, -16, -13, -8, -3, 0, 1, 3, 6, 8, 10, 12, 12 }, + { 10, 10, 10, 10, 10, 10, 10, 9, 9, 6, 0, -6, -14, -20, -22, -22, -19, -15, -10, -6, -2, -2, -4, -7, -8, -8, -7, -4, 0, 1, 1, 2, 4, 6, 8, 10, 10 }, + { 9, 9, 9, 9, 9, 9, 8, 8, 7, 4, -1, -8, -15, -19, -20, -18, -14, -9, -5, -2, 0, 1, 0, -2, -3, -4, -3, -2, 0, 0, 0, 1, 3, 5, 7, 8, 9 }, + { 8, 8, 8, 9, 9, 9, 8, 8, 6, 2, -3, -9, -15, -18, -17, -14, -10, -6, -2, 0, 1, 2, 2, 0, -1, -1, -2, -1, 0, 0, 0, 0, 1, 3, 5, 7, 8 }, + { 8, 9, 9, 10, 10, 10, 10, 8, 5, 0, -5, -11, -15, -16, -15, -12, -8, -4, -1, 0, 2, 3, 2, 1, 0, 0, 0, 0, 0, -1, -2, -2, -1, 0, 3, 6, 8 }, + { 6, 9, 10, 11, 12, 12, 11, 9, 5, 0, -7, -12, -15, -15, -13, -10, -7, -3, 0, 1, 2, 3, 3, 3, 2, 1, 0, 0, -1, -3, -4, -5, -5, -2, 0, 3, 6 }, + { 5, 8, 11, 13, 15, 15, 14, 11, 5, -1, -9, -14, -17, -16, -14, -11, -7, -3, 0, 1, 3, 4, 5, 5, 5, 4, 3, 1, -1, -4, -7, -8, -8, -6, -2, 1, 5 }, + { 4, 8, 12, 15, 17, 18, 16, 12, 5, -3, -12, -18, -20, -19, -16, -13, -8, -4, -1, 1, 4, 6, 8, 9, 9, 9, 7, 3, -1, -6, -10, -12, -11, -9, -5, 0, 4 }, + { 3, 9, 14, 17, 20, 21, 19, 14, 4, -8, -19, -25, -26, -25, -21, -17, -12, -7, -2, 1, 5, 9, 13, 15, 16, 16, 13, 7, 0, -7, -12, -15, -14, -11, -6, -1, 3 }, +}; + +static float get_lookup_table_val(unsigned lat, unsigned lon); + +__EXPORT float get_mag_declination(float lat, float lon) +{ + /* + * If the values exceed valid ranges, return zero as default + * as we have no way of knowing what the closest real value + * would be. + */ + if (lat < -90.0f || lat > 90.0f || + lon < -180.0f || lon > 180.0f) { + return 0.0f; + } + + /* round down to nearest sampling resolution */ + int min_lat = (int)(lat / SAMPLING_RES) * SAMPLING_RES; + int min_lon = (int)(lon / SAMPLING_RES) * SAMPLING_RES; + + /* for the rare case of hitting the bounds exactly + * the rounding logic wouldn't fit, so enforce it. + */ + + /* limit to table bounds - required for maxima even when table spans full globe range */ + if (lat <= SAMPLING_MIN_LAT) { + min_lat = SAMPLING_MIN_LAT; + } + + if (lat >= SAMPLING_MAX_LAT) { + min_lat = (int)(lat / SAMPLING_RES) * SAMPLING_RES - SAMPLING_RES; + } + + if (lon <= SAMPLING_MIN_LON) { + min_lon = SAMPLING_MIN_LON; + } + + if (lon >= SAMPLING_MAX_LON) { + min_lon = (int)(lon / SAMPLING_RES) * SAMPLING_RES - SAMPLING_RES; + } + + /* find index of nearest low sampling point */ + unsigned min_lat_index = (-(SAMPLING_MIN_LAT) + min_lat) / SAMPLING_RES; + unsigned min_lon_index = (-(SAMPLING_MIN_LON) + min_lon) / SAMPLING_RES; + + float declination_sw = get_lookup_table_val(min_lat_index, min_lon_index); + float declination_se = get_lookup_table_val(min_lat_index, min_lon_index + 1); + float declination_ne = get_lookup_table_val(min_lat_index + 1, min_lon_index + 1); + float declination_nw = get_lookup_table_val(min_lat_index + 1, min_lon_index); + + /* perform bilinear interpolation on the four grid corners */ + + float declination_min = ((lon - min_lon) / SAMPLING_RES) * (declination_se - declination_sw) + declination_sw; + float declination_max = ((lon - min_lon) / SAMPLING_RES) * (declination_ne - declination_nw) + declination_nw; + + return ((lat - min_lat) / SAMPLING_RES) * (declination_max - declination_min) + declination_min; +} + +float get_lookup_table_val(unsigned lat_index, unsigned lon_index) +{ + return declination_table[lat_index][lon_index]; +}
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