Silva Fennica

Fig. 1. Mean values of errors (6 repetitions) before calibration, for each hypsometer and height (6 m, 8 m, 10 m and 12 m). Desire = HTC Desire; Note = Samsung Galaxy Note; Height = Tool Height (Smart Tools); Slope = Tool Slope (Smart Tools). The data on the left for each hypsometer (dashed box) correspond to a distance of 10 m; those in the centre to 15 m; and those on the right to 20 m.

Table 1. Modulus of the relative errors of the height measurements of the hypsometers (with and without calibration of the smartphones).
Hypsometer	Cal.	n	Mean (%)	Median (%)	SD (%)	95% CI (%)	nFIA	%FIA
Vertex		72	0.80	0.83	0.86	0.60, 1.00	72	100
Blume Leiss		72	2.96	2.96	1.66	2.58, 3.35	72	100
Desire, Height	N	72	8.38	8.33	1.77	7.94, 8.83	62	86.1 <>
Desire, Slope	N	72	9.24	9.09	1.99	8.68, 9.79	51	70.8 <>
Note, Height	N	72	5.87	6.00	1.26	5.57, 6.16	72	100
Note, Slope	N	72	6.73	6.66	1.13	6.47, 7.00	72	100
Desire, Height	Y	72	1.38	1.27	0.98	1.15, 1.61	72	100
Desire, Slope	Y	72	3.14	3.22	1.74	2.73, 3.55	71	98.6
Note, Height	Y	72	1.18	1.01	1.00	0.95, 1.41	72	100
Note, Slope	Y	72	2.41	2.36	1.03	2.17, 2.65	72	100
Cal. = calibrated (N = no; Y = yes); n = number of height measurements; SD = standard deviation; 95% CI = confidence interval at the 95% confidence level; nFIA = number of measurements that fulfil the FIA criterion; %FIA = percentage of measurements that fulfil the FIA criterion. <> = Does not fulfil the FIA quality criterion.

Table 2. Relative errors of the height measurements of the hypsometers (with and without calibration of the smartphones).
Hypsometer	Cal.	n	Mean (%)	Median (%)	SD (%)	95% CI (%)	RMSE
Vertex		72	–0.25	0.00	1.15	–0.52, 0.02	0.0117
Blume Leiss		72	–2.78	–2.81	1.96	–3.23, –2.32	0.0339
Desire, Height	N	72	–8.38	–8.33	1.77	–8.80, –7.97	0.0856
Desire, Slope	N	72	–9.24	–9.09	1.99	–9.70, –8.77	0.0945
Note, Height	N	72	–5.87	–6.00	1.26	–6.16, –5.57	0.0600
Note, Slope	N	72	–6.73	–6.66	1.13	–7.00, –6.47	0.0683
Desire, Height	Y	72	0.54	0.46	1.61	0.17, 0.92	0.0169
Desire, Slope	Y	72	–3.13	–3.22	1.77	–3.54, –2.72	0.0359
Note, Height	Y	72	–0.61	–0.58	1.42	–0.95, –0.28	0.0154
Note, Slope	Y	72	–2.41	–2.36	1.03	–2.65, –2.17	0.0262
Cal. = calibrated (N = no; Y = yes); n = number of height measurements; SD = standard deviation; 95% CI = confidence interval at the 95% confidence level; RMSE = root-mean-square error.

Fig. 2. Medians of the modulus of the relative errors before calibration. Desire = HTC Desire; Note = Samsung Galaxy Note; Height = Tool Height (Smart Tools); Slope = Tool Slope (Smart Tools). Medians of hypsometers without a common letter are significantly different at p = 0.05.

Fig. 3. Differences between the smartphone readings (measured on axis Z) and those of the Digital Angle Finder (Right angle).

Table 3. Regression models of the smartphones and the Digital Angle Finder.
Smartphone	Axis	Regression model	R²	P-values
Smartphone	Axis	Regression model	R²	Intercept	Coefficient
Note	Y	f(y) = 1.04471y + 0.35464	0.99925	<0.0001	<0.0001
Note	Z	f(z) = 1.03904z + 0.05963	0.99993	0.263	<0.0001
Desire	Y	f(y) = 1.09512y – 1.84003	0.99952	<0.0001	<0.0001
Desire	Z	f(z) = 1.05483z + 0.39165	0.99995	<0.0001	<0.0001
Note = Samsung Galaxy Note; Desire = HTC Desire. y, z = angles obtained with the smartphone. f(y), f(z) = corrected estimated angles.

Fig. 4. Medians of the modulus of the relative errors after calibration. Desire = HTC Desire; Note = Samsung Galaxy Note; Height = Tool Height (Smart Tools); Slope = Tool Slope (Smart Tools). Medians of the hypsometers without a common letter are significantly different at p = 0.05.