From: Keith Packard Date: Fri, 21 Sep 2012 11:29:17 +0000 (+0200) Subject: altos: Shrink Pa to altitude table X-Git-Tag: 1.1.9.1~51 X-Git-Url: https://git.gag.com/?p=fw%2Faltos;a=commitdiff_plain;h=dc7216d286cc7fe8007f5208ad97a630166572f3 altos: Shrink Pa to altitude table This improves the computation of the table enough that errors from a 470 entry table are almost all < 0.5m. Signed-off-by: Keith Packard --- diff --git a/src/util/make-altitude-pa b/src/util/make-altitude-pa index 190b36fc..eae5ebe9 100644 --- a/src/util/make-altitude-pa +++ b/src/util/make-altitude-pa @@ -29,10 +29,10 @@ const real LAYER0_BASE_PRESSURE = 101325; /* lapse rate and base altitude for each layer in the atmosphere */ const real[NUMBER_OF_LAYERS] lapse_rate = { - -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002 + -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002, }; const int[NUMBER_OF_LAYERS] base_altitude = { - 0, 11000, 20000, 32000, 47000, 51000, 71000 + 0, 11000, 20000, 32000, 47000, 51000, 71000, }; @@ -54,7 +54,7 @@ real altitude_to_pressure(real altitude) { /* calculate the base temperature and pressure for the atmospheric layer associated with the inputted altitude */ - for(layer_number = 0; layer_number < NUMBER_OF_LAYERS - 1 && altitude > base_altitude[layer_number + 1]; layer_number++) { + for(layer_number = 0; layer_number < NUMBER_OF_LAYERS - 2 && altitude > base_altitude[layer_number + 1]; layer_number++) { delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number]; if (lapse_rate[layer_number] == 0.0) { exponent = GRAVITATIONAL_ACCELERATION * delta_z @@ -113,7 +113,7 @@ real pressure_to_altitude(real pressure) { /* calculate the base temperature and pressure for the atmospheric layer associated with the inputted pressure. */ layer_number = -1; - do { + while (layer_number < NUMBER_OF_LAYERS - 2) { layer_number++; base_pressure = next_base_pressure; base_temperature = next_base_temperature; @@ -130,8 +130,9 @@ real pressure_to_altitude(real pressure) { next_base_pressure *= pow(base, exponent); } next_base_temperature += delta_z * lapse_rate[layer_number]; + if (pressure >= next_base_pressure) + break; } - while(layer_number < NUMBER_OF_LAYERS - 1 && pressure < next_base_pressure); /* calculate the altitude associated with the inputted pressure */ if (lapse_rate[layer_number] == 0.0) { @@ -148,20 +149,9 @@ real pressure_to_altitude(real pressure) { altitude = base_altitude[layer_number] + coefficient * (pow(base, exponent) - 1); } - return altitude; } -real feet_to_meters(real feet) -{ - return feet * (12 * 2.54 / 100); -} - -real meters_to_feet(real meters) -{ - return meters / (12 * 2.54 / 100); -} - /* * Values for our MS5607 * @@ -174,14 +164,15 @@ real meters_to_feet(real meters) typedef struct { real m, b; - int m_i, b_i; } line_t; +/* + * Linear least-squares fit values in the specified array + */ line_t best_fit(real[] values, int first, int last) { real sum_x = 0, sum_x2 = 0, sum_y = 0, sum_xy = 0; int n = last - first + 1; real m, b; - int m_i, b_i; for (int i = first; i <= last; i++) { sum_x += i; @@ -197,9 +188,10 @@ line_t best_fit(real[] values, int first, int last) { real min_Pa = 0; real max_Pa = 120000; -/* Target is an array of < 2000 entries */ -int pa_sample_shift = 3; -int pa_part_shift = 3; +/* Target is an array of < 1000 entries */ +int pa_sample_shift = 2; +int pa_part_shift = 6; +int pa_part_mask = (1 << pa_part_shift) - 1; int num_part = ceil(max_Pa / (2 ** (pa_part_shift + pa_sample_shift))); @@ -211,6 +203,10 @@ real sample_to_altitude(int sample) = pressure_to_altitude(sample_to_Pa(sample)) int part_to_sample(int part) = part << pa_part_shift; +int sample_to_part(int sample) = sample >> pa_part_shift; + +bool is_part(int sample) = (sample & pa_part_mask) == 0; + real[num_samples] alt = { [n] = sample_to_altitude(n) }; int seg_len = 1 << pa_part_shift; @@ -219,18 +215,22 @@ line_t [num_part] fit = { [n] = best_fit(alt, n * seg_len, n * seg_len + seg_len - 1) }; -int[num_samples/seg_len + 1] alt_part; +real[num_samples/seg_len + 1] alt_part; +real[dim(alt_part)] alt_error = {0...}; -alt_part[0] = floor (fit[0].b + 0.5); -alt_part[dim(fit)] = floor(fit[dim(fit)-1].m * dim(fit) * seg_len + fit[dim(fit)-1].b + 0.5); +alt_part[0] = fit[0].b; +alt_part[dim(fit)] = fit[dim(fit)-1].m * dim(fit) * seg_len + fit[dim(fit)-1].b; for (int i = 0; i < dim(fit) - 1; i++) { real here, there; here = fit[i].m * (i+1) * seg_len + fit[i].b; there = fit[i+1].m * (i+1) * seg_len + fit[i+1].b; - alt_part[i+1] = floor ((here + there) / 2 + 0.5); +# printf ("at %d mis-fit %8.2f\n", i, there - here); + alt_part[i+1] = (here + there) / 2; } +real round(real x) = floor(x + 0.5); + real sample_to_fit_altitude(int sample) { int sub = sample // seg_len; int off = sample % seg_len; @@ -239,7 +239,7 @@ real sample_to_fit_altitude(int sample) { real i_v; r_v = sample * l.m + l.b; - i_v = (alt_part[sub] * (seg_len - off) + alt_part[sub+1] * off) / seg_len; + i_v = (round(alt_part[sub]) * (seg_len - off) + round(alt_part[sub+1]) * off) / seg_len; return i_v; } @@ -249,27 +249,41 @@ real total_error = 0; for (int sample = 0; sample < num_samples; sample++) { real Pa = sample_to_Pa(sample); - real meters = pressure_to_altitude(Pa); + real meters = alt[sample]; real meters_approx = sample_to_fit_altitude(sample); real error = abs(meters - meters_approx); + int part = sample_to_part(sample); + + if (error > alt_error[part]) + alt_error[part] = error; + total_error += error; if (error > max_error) { max_error = error; max_error_sample = sample; } -# printf (" %7d, /* %6.2f kPa %5d sample approx %d */\n", -# floor (meters + 0.5), Pa / 1000, sample, floor(sample_to_fit_altitude(sample) + 0.5)); + if (false) { + printf (" %8.1f %8.2f %8.2f %8.2f %s\n", + Pa, + meters, + meters_approx, + meters - meters_approx, + is_part(sample) ? "*" : ""); + } } -printf ("/*max error %f at %7.3f%%. Average error %f*/\n", max_error, max_error_sample / (num_samples - 1) * 100, total_error / num_samples); +printf ("/*max error %f at %7.3f kPa. Average error %f*/\n", + max_error, sample_to_Pa(max_error_sample) / 1000, total_error / num_samples); printf ("#define NALT %d\n", dim(alt_part)); printf ("#define ALT_SHIFT %d\n", pa_part_shift + pa_sample_shift); +printf ("#ifndef SATURATE\n#define SATURATE(x) (x)\n#endif\n"); for (int part = 0; part < dim(alt_part); part++) { real kPa = sample_to_Pa(part_to_sample(part)) / 1000; - printf ("%9d, /* %6.2f kPa */\n", - alt_part[part], kPa); + printf ("SATURATE(%9d), /* %6.2f kPa error %6.2fm */\n", + round (alt_part[part]), kPa, + alt_error[part]); }