Understanding the proportions and accuracy of Bing Map

 

Enjoy yourself at home during the National Day. Article To translate and share with you. Today we will talk about the scale and latitude and longitude of the map. In fact, I used to have this problem. We often mention that the accuracy of the satellite images provided by Bing Maps in Beijing is 1 M. What does this mean? What do we usually see in the lower right corner of a map? We all know that the Earth is a circle. To be exact, it is not a rule of the world. The electronic maps we see every day, including paper maps, are flat. For convenience, we must map the irregular sphere to a plane at the expense of certain accuracy. This is the most common Mercator projection we see now ), almost all electronic maps, including Bing Maps, use Mercator projection. The advantage of using Mercator projection is that, if we sail on the sea, we follow the instructions of the compass to keep moving forward, and the corresponding line is also a line on the planimetric map, although the sphere actually follows a spatial curve. Of course, Mercator projection also has problems, because it uses parallel longitude and latitude lines, which means that the distance between the longitude and latitude lines is equal. However, on the Earth sphere, the distance between the equator and the pole is obviously not the same. The advantage of using Mercator projection is that it makes it easier for us to draw and understand maps. [How to calculate resolution]

The resolution of a map is an equation composed of latitude, zoom-in level, and constants. At any latitude or magnification level, the map accuracy can be expressed by the following equations:

Map resolution = 156543.04 meters/pixel * Cos (latitude)/(2 ^ zoomlevel)

Here, the zoomlevel value is an integer between 1 and 19, and the latitude is-90 to + 90. The closer the two poles, the greater the error of this equation. This is the problem caused by Mercator projection. From this equation, we can calculate the ratio of distance represented by each pixel on the screen (on the equator) at each zoom level ):

It should be noted that the ratio varies under different latitudes. For example, in Toronto (Toronto, dimension 43.65), the map zoom level is 13, and the map accuracy is about19.11 * Cos (43.65) or13.8 meters/pixel.

[How to calculate the scale]

Most of the time, we pay more attention to the map scale, because we are more accustomed to a fixed proportion of the map, such as, that is, 1 cm on the map corresponds to 50 on the earth. To calculate the scale, you need to know the screen resolution, map zoom level, and dimension value. In addition, we assume that the screen resolution is fixed and is equal in the direction of X and Y. The equation for scale calculation is as follows:

Map scale = 1: (screenres pixels/inch * 39.37 inches/meter * 156543.04 meters/pixel * Cos (latitude)/(2 ^ zoomlevel ))

For example, if the screen resolution is 85/inch and the map zoom level is 13, you can obtain a scale of 1: 85*39.37*19.11 or 1: 63950.

Although most of the time we do not need to calculate the scale or map accuracy, this is usually done by Microsoft Bing map in the background. However, you may need to consider when developing some applications, such as vehicle navigation or distance calculation.