Shadow length at noon from winter solstice in Ningbo 1 m

The shadow length of a one-meter-high building from winter to the sun is1.92m (taking Beijing as an example).

Shadow length L=Hcoth, h is the height of the building, and h is the elevation angle of the sun. H=90-|φ-δ|, φ is the local latitude, δ is the solar declination, that is, the latitude where the direct point of the sun is located (positive value in the northern hemisphere). For example, when Beijing is located at 39 degrees north latitude and winter solstice, h=90-|39+23.5|=27.5 degrees, L=Hcoth= 1.92 meters.

The change of latitude and longitude will affect the measurement error, so the geographical location of the city or the shape and outline of the city will directly affect the variation law of the measurement error. Using satellite images as source data, the weather and time of image formation should be considered when obtaining shadow information. Different times of the year and day will lead to different shadow lengths.

For example, every morning, evening or winter, the shadow of the image will be very long, resulting in the shadow being blocked by buildings or other ground objects, resulting in the incomplete shadow. Similarly, every noon or summer, the shadow will be very short, which will increase the error of manual interpretation. Therefore, appropriate image data should be selected as required.