Averaging and sampling for magnetic-observatory hourly data
Averaging and sampling for magnetic-observatory hourly data
Blog Article
A time and frequency-domain analysis is made of the effects of averaging and sampling methods used for constructing magnetic-observatory hourly data values.Using 1-min data as a proxy for continuous, geomagnetic variation, we construct synthetic hourly values of two standard types: instantaneous "spot" measurements and simple 1-h "boxcar" averages.We compare these average-sample types with others: 2-h average, Gaussian, and "brick-wall" low-frequency-pass.Hourly spot measurements provide a statistically unbiased representation of the amplitude range of geomagnetic-field variation, but as a representation of continuous field variation over time, they are significantly affected by aliasing, especially at high latitudes.The 1-h, 2-h, and Gaussian average-samples bekindtopets.com are affected by a combination of amplitude distortion and aliasing.
Brick-wall values are not affected by either amplitude distortion or aliasing, but constructing them is, in an operational setting, relatively more difficult than it is for other average-sample types.It is noteworthy that 1-h average-samples, the present standard for observatory hourly data, have properties similar to Gaussian average-samples that have been optimized for a minimum residual sum of amplitude distortion and aliasing.For 1-h average-samples from medium and low-latitude observatories, the average of the combination of amplitude distortion and aliasing is less than the 5.0 nT accuracy standard established by Intermagnet for modern 1-min data.For medium and low-latitude observatories, average differences between monthly means constructed from 1-min data and monthly means constructed from any of the hourly average-sample types considered here are less color block iphone case than the 1.
0 nT resolution of standard databases.We recommend that observatories and World Data Centers continue the standard practice of reporting simple 1-h-average hourly values.