I have soil moisture data with x-, y- and z-coordinates like this:
gue <- structure(list(x = c(311939.1507, 311935.4607, 311924.7316, 311959.553,
311973.5368, 311953.3743, 311957.9409, 311948.3151, 311946.7169,
311997.0803, 312017.5236, 312006.0245, 312001.5179, 311992.7044,
311977.3076, 311960.4159, 311970.6047, 311957.2564, 311866.4246,
311870.8714, 311861.4461, 311928.7096, 311929.6291, 311929.4233,
311891.2915, 311890.3429, 311900.8905, 311864.4995, 311870.8143,
311866.9257, 312002.571, 312017.816, 312004.5024, 311947.1186,
311943.0152, 311952.2695, 311920.6095, 311929.8371, 311918.6095,
312011.9019, 311999.5755, 312011.1461, 311913.7251, 311925.3459,
311944.4701, 311910.2079, 311908.7618, 311896.0776, 311864.4814,
311856.9027, 311857.5747, 311967.3779, 311962.2024, 311956.8318,
311977.5254, 311971.1776, 311982.537, 311993.4709, 312004.6407,
312015.6118, 311990.8601, 311994.686, 311988.3037, 311990.518,
311986.3918, 311998.8876, 311923.9157, 311903.4563, 311915.714,
311856.9087, 311858.9812, 311874.5867, 311963.9099, 311938.4542,
311945.9505, 311804.3039, 311797.2571, 311791.6967, 311921.3965,
311928.9353, 311920.0597, 311833.5109, 311829.8683, 311847.6261,
311889.1243, 311902.4909, 311901.245, 311981.1118, 312005.7098,
311976.5858, 311819.8901, 311816.4143, 311819.4172, 311870.418,
311873.2656, 311888.3401, 311910.8377, 311897.6697, 311902.4571,
311846.8196, 311833.6235, 311846.2942, 311931.3916, 311930.1891,
311947.659, 311792.2642, 311793.2539, 311794.1931, 311795.1288,
311796.0806, 311797.0142, 311797.95, 311798.8822, 311799.8229,
311800.7774, 311801.7094, 311802.6395, 311803.583, 311804.5185,
311805.4558, 311806.391, 311807.3346, 311808.2757, 311809.2187,
311810.1549, 311811.1014, 311812.0366, 311812.9667, 311813.9107,
311814.8373, 311815.7777, 311816.7365, 311817.6522, 311818.6091,
311819.5335, 311820.4961, 311821.4337, 311822.3855, 311823.3195,
311824.2713, 311825.214, 311826.1705, 311827.1188, 311828.0501,
311828.9893, 311829.9324, 311830.8706, 311831.8181, 311832.7667,
311833.705, 311834.6546, 311835.609, 311836.5527, 311837.5157,
311838.4495, 311839.3926, 311840.3423, 311841.2799, 311842.2288,
311843.1691, 311844.118, 311845.0746, 311846.019, 311846.9709,
311847.9201, 311848.859, 311849.8105, 311850.7503, 311851.6889,
311852.6355, 311853.6045, 311854.5296, 311855.4717, 311856.4171,
311857.3759, 311858.3151, 311859.2604, 311860.2178, 311861.1636,
311862.1071, 311863.0347, 311863.9857, 311864.9316, 311865.8722,
311866.8158, 311867.7702, 311868.7155, 311869.649, 311870.6018,
311871.5449, 311872.4871, 311873.4352, 311874.385, 311875.3042,
311876.2617, 311877.2068, 311878.1429, 311879.0956, 311880.0401,
311880.9822, 311881.929, 311882.8651, 311883.8017, 311884.7429,
311885.6949, 311886.6349, 311887.7207, 311888.6653, 311889.6041,
311890.5358, 311891.4838, 311892.4292, 311893.3736, 311894.326,
311895.2703, 311896.2182, 311897.1635, 311898.1032, 311899.0496,
311899.9967, 311900.9456, 311901.8889, 311902.8162, 311903.7566,
311904.6996, 311905.6627, 311906.5899, 311907.5448, 311908.4856,
311909.4399, 311910.3649, 311911.3188, 311912.2629, 311913.2022,
311914.1527, 311915.1025, 311916.0425, 311916.985, 311917.9254,
311918.8661, 311919.8174, 311920.7668, 311921.7026, 311922.6517,
311923.5949, 311924.5252, 311925.4599, 311926.422, 311927.3646,
311928.3, 311929.2432, 311930.1796, 311931.1358, 311932.0726,
311933.0069, 311933.9585, 311934.845, 311935.7788, 311936.7193,
311937.6441, 311938.572, 311939.5094, 311940.4666, 311941.4067,
311942.3489, 311943.2712, 311944.2195, 311945.1536, 311946.0927,
311947.0413, 311947.9761, 311948.9082, 311949.8557, 311950.8201,
311951.7616, 311952.7148, 311953.7894, 311954.7289, 311955.6646,
311956.6081, 311957.5588, 311958.4896, 311959.4297, 311960.3761,
311961.3191, 311962.2653, 311963.195, 311964.1501, 311965.0856,
311966.0254, 311966.9739, 311967.9305, 311968.8592, 311971.7861,
311970.758, 311969.8205), y = c(5846548.408, 5846546.489, 5846538.014,
5846525.283, 5846510.302, 5846503.516, 5846529.769, 5846523.06,
5846522.742, 5846512.263, 5846525.347, 5846522.042, 5846537.487,
5846545.587, 5846532.112, 5846425.917, 5846406.543, 5846434.03,
5846500.989, 5846498.286, 5846487.134, 5846488.045, 5846483.29,
5846468.713, 5846534.269, 5846533.527, 5846504.056, 5846453.395,
5846438.43, 5846442.608, 5846406.8, 5846434.58, 5846419.229,
5846441.045, 5846436.903, 5846447.917, 5846460.757, 5846457.428,
5846451.067, 5846445.596, 5846474.031, 5846457.239, 5846532.694,
5846553.938, 5846565.323, 5846446.926, 5846432.549, 5846467.236,
5846473.963, 5846464.78, 5846498.142, 5846458.168, 5846474.018,
5846489.801, 5846559.513, 5846589.975, 5846555.723, 5846553.847,
5846560.066, 5846560.792, 5846455.642, 5846546.374, 5846465.999,
5846432.091, 5846422.061, 5846442.871, 5846485.956, 5846472.811,
5846506.756, 5846416.327, 5846419.623, 5846413.124, 5846587.334,
5846600.116, 5846589.515, 5846463.69, 5846456.712, 5846459.683,
5846600.118, 5846574.99, 5846597.804, 5846419.496, 5846437.615,
5846436.902, 5846567.872, 5846572.857, 5846556.904, 5846388.146,
5846393.088, 5846390.13, 5846481.09, 5846496.127, 5846493.586,
5846545.396, 5846532.126, 5846538.334, 5846388.343, 5846416.117,
5846392.223, 5846513.526, 5846486.644, 5846512.917, 5846395.509,
5846386.421, 5846383.873, 5846459.062, 5846459.36, 5846459.682,
5846460.026, 5846460.377, 5846460.703, 5846461.047, 5846461.378,
5846461.73, 5846462.071, 5846462.418, 5846462.765, 5846463.115,
5846463.466, 5846463.815, 5846464.128, 5846464.505, 5846464.843,
5846465.189, 5846465.52, 5846465.869, 5846466.217, 5846466.557,
5846466.893, 5846467.237, 5846467.586, 5846467.903, 5846468.274,
5846468.601, 5846468.943, 5846469.258, 5846469.592, 5846469.909,
5846470.247, 5846470.565, 5846470.891, 5846471.24, 5846471.536,
5846471.885, 5846472.224, 5846472.553, 5846472.884, 5846473.225,
5846473.532, 5846473.89, 5846474.179, 5846474.502, 5846474.827,
5846475.146, 5846475.448, 5846475.768, 5846476.102, 5846476.428,
5846476.746, 5846477.069, 5846477.37, 5846477.685, 5846478.009,
5846478.335, 5846478.656, 5846478.958, 5846479.299, 5846479.608,
5846479.926, 5846480.267, 5846480.603, 5846480.908, 5846481.246,
5846481.56, 5846481.877, 5846482.19, 5846482.503, 5846482.825,
5846483.144, 5846483.468, 5846483.811, 5846484.13, 5846484.458,
5846484.8, 5846485.125, 5846485.456, 5846485.778, 5846486.112,
5846486.421, 5846486.75, 5846487.08, 5846487.401, 5846487.744,
5846488.067, 5846488.39, 5846488.728, 5846489.067, 5846489.383,
5846489.716, 5846490.054, 5846490.38, 5846490.719, 5846491.044,
5846491.357, 5846491.694, 5846492.005, 5846492.402, 5846492.726,
5846493.045, 5846493.389, 5846493.708, 5846494.049, 5846494.363,
5846494.686, 5846494.982, 5846495.3, 5846495.64, 5846495.957,
5846496.263, 5846496.584, 5846496.911, 5846497.241, 5846497.591,
5846497.914, 5846498.226, 5846498.553, 5846498.893, 5846499.221,
5846499.538, 5846499.869, 5846500.19, 5846500.508, 5846500.82,
5846501.151, 5846501.492, 5846501.827, 5846502.147, 5846502.471,
5846502.803, 5846503.129, 5846503.46, 5846503.783, 5846504.11,
5846504.448, 5846504.76, 5846505.118, 5846505.445, 5846505.79,
5846506.106, 5846506.465, 5846506.795, 5846507.118, 5846507.448,
5846507.758, 5846508.081, 5846508.396, 5846508.645, 5846508.99,
5846509.34, 5846509.685, 5846510.031, 5846510.363, 5846510.693,
5846511.031, 5846511.362, 5846511.694, 5846512.024, 5846512.354,
5846512.701, 5846513.034, 5846513.353, 5846513.683, 5846513.998,
5846514.32, 5846514.636, 5846514.956, 5846515.326, 5846515.65,
5846515.968, 5846516.301, 5846516.634, 5846516.971, 5846517.318,
5846517.64, 5846517.952, 5846518.308, 5846518.626, 5846518.937,
5846519.27, 5846519.597, 5846519.921, 5846520.245, 5846520.581,
5846521.498, 5846521.209, 5846520.893), z = c(26.485, 26.411,
26.339, 27.248, 27.208, 26.799, 27.199, 27.023, 26.973, 26.908,
26.275, 26.474, 26.316, 26.226, 27.184, 25.903, 25.765, 25.931,
26.057, 26.181, 26.102, 26.436, 26.457, 26.396, 25.585, 25.572,
26.448, 25.637, 25.603, 25.634, 25.847, 26.185, 25.899, 26.016,
25.873, 26.299, 26.358, 26.344, 26.088, 26.264, 26.3, 26.306,
26.311, 25.857, 26.004, 25.824, 25.798, 26.326, 26.03, 25.625,
25.78, 26.368, 26.225, 26.582, 26.398, 25.343, 26.253, 25.908,
25.323, 25.381, 26.3, 26.179, 26.284, 26.024, 25.896, 26.251,
26.447, 26.385, 26.419, 25.188, 25.176, 25.169, 25.348, 25.188,
25.291, 25.285, 25.266, 25.262, 25.333, 25.308, 25.314, 25.145,
25.172, 25.22, 25.235, 25.204, 25.286, 25.155, 25.397, 25.202,
25.373, 25.327, 25.341, 25.172, 25.253, 25.318, 25.023, 25.24,
25.132, 25.264, 25.38, 25.221, 25.119, 25.179, 25.083, 25.258,
25.254, 25.235, 25.252, 25.266, 25.256, 25.264, 25.26, 25.262,
25.265, 25.265, 25.285, 25.28, 25.257, 25.254, 25.258, 25.287,
25.294, 25.282, 25.27, 25.268, 25.309, 25.303, 25.3, 25.312,
25.305, 25.3, 25.314, 25.319, 25.328, 25.304, 25.325, 25.308,
25.332, 25.333, 25.333, 25.346, 25.344, 25.339, 25.355, 25.362,
25.36, 25.391, 25.418, 25.434, 25.436, 25.447, 25.486, 25.5,
25.526, 25.552, 25.551, 25.564, 25.589, 25.606, 25.641, 25.672,
25.689, 25.709, 25.736, 25.758, 25.782, 25.836, 25.844, 25.866,
25.88, 25.935, 25.984, 26.037, 26.066, 26.071, 26.094, 26.106,
26.106, 26.118, 26.1, 26.146, 26.135, 26.156, 26.169, 26.162,
26.173, 26.198, 26.196, 26.228, 26.258, 26.276, 26.283, 26.277,
26.236, 26.277, 26.251, 26.264, 26.26, 26.261, 26.249, 26.307,
26.289, 26.243, 26.206, 26.231, 26.224, 26.238, 26.244, 26.245,
26.254, 26.2, 26.229, 26.24, 26.248, 26.223, 26.29, 26.344, 26.371,
26.364, 26.311, 26.343, 26.342, 26.334, 26.317, 26.342, 26.315,
26.312, 26.322, 26.325, 26.324, 26.32, 26.308, 26.329, 26.31,
26.32, 26.327, 26.34, 26.371, 26.442, 26.442, 26.483, 26.504,
26.526, 26.562, 26.562, 26.538, 26.534, 26.533, 26.541, 26.584,
26.642, 26.65, 26.691, 26.719, 26.755, 26.786, 26.794, 26.849,
26.867, 26.919, 26.93, 26.945, 26.947, 26.959, 26.984, 26.992,
27.006, 27.035, 27.021, 27.052, 27.094, 27.104, 27.119, 27.16,
27.182, 27.223, 27.236, 27.267, 27.304, 27.331, 27.348, 27.341,
27.379, 27.355, 27.378, 27.357, 27.373, 27.319, 27.299, 27.278,
27.28, 27.295, 27.288, 27.286, 27.279), soil_m_sat = c(24.1,
24.2, 26.9, 13.9, 20.6, 34.1, 16.2, 16.7, 16, 22.1, 23.9, 27.2,
26.8, 34.4, 26.3, 54.1, 51, 44.9, 46.4, 45.9, 54.7, 39.1, 38.7,
40.7, 56.5, 56.3, 40.6, 60.9, 56.8, 56.3, 40.7, 40.4, 44.1, 44.9,
46.2, 45.3, 46.1, 43.7, 44.9, 45.4, 33.1, 45.8, 27.6, 47.8, 37.3,
58.9, 51.4, 42.1, 46, 66.6, 51.1, 31.6, 48.7, 32.9, 28.1, 84,
37.7, 38.2, 80.4, 73.3, 35.6, 44.2, 39.7, 50.2, 49.9, 37.8, 37,
41.7, 27.3, 100, 100, 100, 80.9, 100, 88.4, 89.6, 93.8, 95.3,
91.9, 93.9, 96.1, 91.4, 100, 94.4, 100, 100, 80, 94.1, 84.4,
91.1, 80, 78.9, 85.9, 100, 97.5, 87.2, 88.6, 83.3, 90.7, 100,
82.2, 100, 96.3, 93.3, 99.6, 92.1, 92.8, 90.9, 92.3, 91.2, 94.5,
91.8, 89.4, 87, 86, 88, 83.7, 88.8, 92.9, 89.3, 83.3, 83.5, 84.5,
85.8, 87.4, 86.5, 82, 78.1, 85.8, 85.6, 88.7, 87.7, 84.9, 82,
87.9, 85.5, 86, 82, 83, 88.5, 81.2, 81.6, 76.5, 77.6, 84.5, 81.5,
82, 82.4, 68, 67.7, 62.1, 68.9, 61.7, 68.5, 68.6, 65.3, 59.5,
60.8, 67.3, 66.2, 59.9, 50.9, 46.9, 44.6, 47.9, 53, 52.1, 48.3,
41.3, 53.8, 51, 47, 53.7, 49.5, 51.1, 44.4, 35.1, 42.2, 41.5,
40, 48.2, 46.7, 48.6, 51.7, 51.2, 52.3, 53.4, 48.9, 50.7, 48.5,
46.5, 39.4, 38, 49.2, 43.6, 47.1, 40.4, 44.7, 45.7, 38.1, 41.9,
39.3, 40.2, 43.8, 47.3, 50.1, 41.2, 39.8, 46, 40.8, 40, 37.8,
42.6, 46, 43.8, 45.4, 42.2, 46.5, 40.4, 39.9, 53, 44.7, 35.8,
42.9, 43.9, 43.2, 40.6, 40.8, 32.2, 32.6, 33.5, 36.7, 34.6, 34.7,
50.9, 35.6, 34.2, 28.1, 42, 32, 42.3, 30, 29.6, 31, 29.8, 26,
37.8, 40, 37, 30.2, 28.2, 26.2, 27.4, 22.1, 28.4, 23.2, 24.8,
26.5, 23.9, 21.1, 27.2, 20.8, 12.5, 14, 17.9, 19.7, 19.4, 26,
16.7, 18.2, 23.9, 19, 25.9, 24.4, 22.1, 19.2, 18.4, 24.7, 17.3,
19.4, 19.6, 17.7, 21.3, 22.1, 17.9, 28.2, 16.3, 25.3, 19.7, 21.7,
19, 18.8, 11.8, 15.6, 9.8, 17.7)), .Names = c("x", "y", "z",
"soil_m_sat"), class = "data.frame", row.names = c(NA, -296L))
In order to estimate a variogram for this data I need to remove the spatial trend from it. The soil moisture, of course, varies with the surface - the higher a point is the dryer it is. And since this soil moisture data is percetagewise the relationship is hardly linear, what leads me to allow up to cubic dependencies of the soil moisture to the z-coordinate. It happens that in this area there is a small more or less elliptic elevation, so that I want to allow the soil moisture to be dependend of the x- and y-coordinates in a quadratic way. I hope the following model does exactly this:
polymod <- lm(soil_m_sat ~ poly(x + y, degree = 2) + poly(z, degree = 3), data = gue)
summary(polymod)
The summary shows me that there is no significance for the first coefficient of the x- and y-dependency (what summary names poly(x + y, degree = 2)1). Because the help page from poly() told me that it "returns or evaluates orthogonal polynomials of degree 1 to degree", I thought, removing a degree one polynom from the model might be the same as removing the first coefficient of the degree 2 polynom. Therefore I tried to remove it like this:
mod <- lm(soil_m_sat ~ poly(x + y, degree = 2) - poly(x + y, degree = 1) + poly(z, degree = 3), data = gue)
summary(mod)
But the summary of mod looks exactly the same as the summary of polymod, meaning mod does not differ from polymod. How is it possible to remove the unsignificant component then?
No, don't check with summary in this case. You should use anova. A polynomial term from poly(), or a spline term from bs() contains more than coefficients, so they are more like a factor variable with multiple levels.
> anova(polymod)
Analysis of Variance Table
Response: soil_m_sat
Df Sum Sq Mean Sq F value Pr(>F)
poly(x + y, degree = 2) 2 113484 56742 1600.8 < 2.2e-16 ***
poly(z, degree = 3) 3 68538 22846 644.5 < 2.2e-16 ***
Residuals 290 10280 35
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
The ANOVA table clearly shows that you need all model terms. Do not drop any.
But I still need to answer your question and make you feel happy.
It is not impossible to drop the poly(x + y, degree = 2)1 term, but you need to access model matrix for such purpose. You may do
gue$XY_poly <- with(gue, poly(x + y, degree = 2))[, 2] ## use the 2nd column only
fit <- lm(soil_m_sat ~ XY_poly + poly(z, degree = 3), data = gue)
summary(fit)
## ...
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 52.3071 0.3459 151.217 < 2e-16 ***
XY_poly -18.8515 7.3894 -2.551 0.0112 *
poly(z, degree = 3)1 -418.1634 6.4937 -64.395 < 2e-16 ***
poly(z, degree = 3)2 116.5327 6.9171 16.847 < 2e-16 ***
poly(z, degree = 3)3 -28.7773 5.9517 -4.835 2.16e-06 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Residual standard error: 5.951 on 291 degrees of freedom
Multiple R-squared: 0.9464, Adjusted R-squared: 0.9457
F-statistic: 1285 on 4 and 291 DF, p-value: < 2.2e-16