The so-called indirect measurement means that after measuring the equipment, the required value can be obtained by calculation, conversion and other methods.
There is no complete direct relationship between them, and the main source of measurement accuracy is composed of comprehensive factors such as method error, measurement equipment accuracy error, human error and environmental error.
The following is a comparison between "manual speed measurement" and "GPS speed measurement".
(1) Manual speed measurement, the most basic speed measurement method is "distance ÷ time". We need to measure these two values and then calculate them. So where does the error come from?
(1), tape measure (or other ranging tools, the same below) have errors in their own accuracy, and various tools can't fully meet the requirements of international standards. There must be discrepancies. If the actual length of your tape measure per meter is 0.999 ~ 1.00 1 m, the error will be 0.1.
(2) The accuracy of the scale value of the tape measure is limited. Generally, for a tape measure of tens of meters, the minimum scale is cm, and at most an estimated value is reserved below cm. If the distance of 100 m is measured, the calibration error is 0.01%(1cm/100 m);
(3) Due to the influence of thermal expansion and contraction, the actual length of tape measure is different in winter and summer. Taking polyester fiber as an example, the thermal expansion coefficient is about 5ppm/℃. If the local temperature difference between winter and summer is 40℃, there is a seasonal error of 0.0 1%.
(4) The stopwatch itself has a precision error. The accuracy of mechanical watches is low, with an error of several minutes a day, but the accuracy of electronic watches is generally high, with an error of only a few seconds or even less than 1 second. Taking 1 s as an example, our precision error is about 0.00 1%.
⑤. In this measurement method, human error accounts for a large proportion. For measuring distance, if the tape measure is tightened and loosened, the error is about 0. 1%. For the measurement time, the average person's reaction time is 0. 1 sec. Assuming that the speed is about10m/s, it takes10s to run100m, and the human error reaches 1%.
6. Calculation error, which depends on the accuracy of numerical values and calculation tools. Now we use electronic calculators to calculate, the accuracy can reach about 10 digits, and the error is only 0.0000000 1%. Calculation error is not an error, it is an error. )
Based on the above factors, when we use v=S/t to calculate the speed:
The error of the distance s is: δ s = 0.1%+0.01%+0.01%+0.1%= 0.22%;
The error of time t is: δ t = 0.001%+1%=1.001%;
The comprehensive error is (1+0.22%) × (1+1.001%)-1=1.2232022%;
The calculation error needs to be added:1.2232022%+0.000001%=1.5438+0%.
This is the final error of this system: 1.438+0%.
(2) GPS speed measurement can be directly read by equipment. Because I don't have the error data of GPS equipment, I can't judge, but it should be much lower than tape measure+stopwatch speed measurement. Of course, it also depends on the data accuracy of GPS velocimeter. For example, we often use mobile phone GPS navigation, and its data accuracy is only kept to an integer (km/h), so this accuracy is relatively low.
But here we should give some error source analysis of GPS speed measurement:
First of all, we must know the principle of GPS speed measurement. In fact, it also uses the "distance-time" method to get the speed value, and the distance measurement method in its system is more complicated.
(1) The GPS velocimeter sends signals to multiple GPS satellites. Because the position and distance of each GPS satellite are different, the time of receiving the signal has priority, and the satellite records the time of receiving the signal;
② The computer needs to calculate the position of the GPS velocimeter according to these time differences and the current position of each satellite. (Before calculation, the computer needs to exclude the reaction time of GPS velocimeter and GPS satellite, which is generally preset in the computer. )
(3) aft that satellite receives the signal sent by the velocimeter for the second time, it still process it in this way to get two positions and two times, and the computer calculates the distance between the two positions, then divides it by the difference between the two times and then divides it by the two times to get the speed value.
Therefore, the speed measurement mechanism of GPS velocimeter is actually more complicated than manual speed measurement, with more calculations and greater errors. However, due to the high precision of the equipment, the comprehensive error will be less than the error of manual speed measurement.
From the speed measuring principle of GPS velocimeter, we know that the so-called "direct measurement" of speed is actually an indirect measurement after disassembly, but because the measuring tools and calculation methods of each link have solidified, the comprehensive error has also solidified, so we classify it as "direct measurement" and only regard the solidified comprehensive error as the equipment error in accurate calculation.
Back to your question:
If you can do it:
(1), use a laser locator with high enough accuracy to measure the distance;
② Use a satellite-like cesium isotope clock to measure time;
(3) connecting them with a computer to obtain the starting position and time, and obtaining the ending position and time, so as to eliminate human error;
(4) Use a high-precision computer program to calculate the data.
Then, the speed you measured will be more accurate than the GPS speedometer.
The reason is very simple: using laser locator to measure the position, the positioning accuracy is micron to millimeter within 100 meters; However, the accuracy of GPS positioning ground position can only reach decimeter ~ meter level.