l Advantages of the GPS system
1.1 high positioning accuracy. The application practice has proved that the relative positioning accuracy of GPS can reach 6-10m within 50kin and 7-10m from 100-500km. 1 000km up to 9---10m.
1.2 The mapping time is short. With the continuous improvement of the GPS system and the continuous updating of the software, at present, relatively static positioning within 20km, only 15-20min, fast static relative positioning measurement, only 1-2min.
1.3 There is no need to look at the measurement station, just open the station overhead, which can save a lot of mapping time and greatly improve work efficiency.
1.4 easy to operate. With the increasing degree of GPS automation, the size of the receiver is getting smaller and smaller, and the weight is getting lighter. Generally, the handheld GPS has a weight of less than 400g.
1.5 work all day. GPS applications are not affected by the climate and can guarantee reception performance.
2 Application of GPS in Forestry Area Surveying and Mapping
The application of GPS global positioning system in forestry area mapping mainly has two aspects. First, it is used to locate the afforestation land, the location and shape of the land, and it is convenient to mark the specific location on the topographic map. The second is to use GPS to calculate the area.
2.1 The determination of the parameters of the central warp of the GPS. Before using GPS measurements, you should first know the central meridian of the area (dividing the integer part of the local longitude by 6 and then taking the integer part of the quotient plus l, multiplying the result by 6 and subtracting 3) to get the local center. The value of the warp value) projection ratio (the value is 1) east-west deviation (the value is 500 000), and the deviation between north and south (the value is 0). Only by entering the correct parameters can the measured data be accurate.
2.1.1 Select the coordinate system. GPS has a variety of coordinate systems, commonly used kilometers network coordinates in China. Therefore, the user should select the custom coordinates before use, and set the format of the standard bit to the user-defined "User Grid".
2.1.2 Parameter setting and input. The parameter "LONGITUDEORIGIN" is the central meridian longitude of the user's location. "SCALE" is the proportional parameter and should be entered "1.0000000"; "ALSEE" input parameter "500000.00"; "FALSEN" input parameter "0.O"; Set the coordinate system to "USER'', where: DX, DY, DZ values, have different values ​​in different work areas, you can get 54 or 84 coordinates of the standard deviation in the local surveying and mapping department, calculate the parameter value and enter GPS DA input k--108, DF input is O.0000005, the input parameters must be verified by the provincial surveying and mapping department.
2.2 The actual application. Measuring area with GPS, measuring distance
2.2.1 Measurement area After multiple comparison tests, use GPS and compass to measure various area plots respectively. The larger the contrast area is, the smaller the gap is and the smaller the area is, the larger the gap between the two is. See the table.
For comprehensive comparison, if the area is larger than lhm2, the measurement is suitable to be measured by GPS. The area below lhm2 should be measured using a compass or a rope. For irregular block measurement area, GPS track measurement method is applied. For regular plots, it is appropriate to use route measurements (GPS measures the area of ​​a polygon surrounded by these vertices, with no more than 50 waypoints per route). When measuring the area, the two methods can be used together to give full play to the effectiveness of GPS.
2.2.2 Ranging. When measuring the distance (the length of the side of a rectangular or square parcel, the length of the shelterbelt), we can use the GPS navigation function to measure the length of the side or the forest belt. First, we set a point at the side length or the forest belt, and navigate at the other end to another point. You can measure the distance.
3 Matters needing attention when using GPS
3.1 Before GPS is used, the necessary inspection should be done. After normal startup, it is automatically set to the factory value. If it is displayed as other values, it should be re-adjusted to the factory value before use. In addition, the necessary error measurement must be made. If the average error exceeds 10m, this machine should not be used.
3.2 Use of common pages. Use the satellite status page when receiving signals after power on. After the signal is stable, use the compass navigation page under the waypoint function in the function menu page, but the reading should be performed after the data is stable, because the GPS receives satellite signals.
The signal will change at any time during the trip, so after reaching the end point, the data should be stable before reading, in order to improve the accuracy of the data.
3.3 Avoid use near obstructions and high voltage wires and transformers. Obstacles affect the reception of satellite signals, the error is often larger, more than 10m, or even more than 20m, therefore, should be avoided in places where there are obstacles to read, especially when the measured distance is short, it should pay attention to this Happening. When there are obstacles in the fixed point, the machine should be as far as possible in a relatively open place, so that the signal is accepted accurately. The magnetic field generated by the high-voltage electricity interferes with the GPS signal, and the difference must be taken when the electromagnetic interference is fixed. The fixed point is to shift the fixed point position by a few meters to avoid electromagnetic interference, and it should be noted in the sketch and reset when the official drawing.
3.4 Prevent data loss. In daily work, data loss may occur due to a variety of reasons. At this time, you can edit the waypoint to regain data.
3.5 The use of GPS mapping area is relatively simple, hand-held GPS along the land to be measured to take a closed track, you can obtain the area, but in order to ensure accuracy in the specific operation, should be maintained at a uniform speed, not too fast.
Shi Yuanning, Hao Yulan (Qinghai Academy of Agriculture and Forestry, Xining 810016)
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