A map that represents the number and characteristics of drawing objects with a set of lines with equal values. Isogram for short. Such as annual average temperature map, annual precipitation map, etc. It is an important mode of thematic map, which was originally used to describe the terrain. Common contour maps and isobathymetric maps show topography and geomorphological structure; Isothermal diagram showing changes of air temperature, water temperature and ground temperature; Isoprecipitation chart showing the change of atmospheric precipitation; Isomagnetic deflection diagram, isomagnetic line diagram and isoseismic line diagram showing geomagnetic and earthquake changes. In addition, there are isobars, isobars, isobars, isobars, isobars, isobars, isobars, isobars, isobars and isobars. Usually, the numerical values represented by isolines are integers.

The preparation of isoline map is usually to mark the values of the relative points (stations) of the drawing object on the geographical map, and then connect the points with equal values into a smooth curve to outline the spatial structure characteristics of the drawing object. Contour maps are often supplemented by layered coloring to improve the intuitive effect of the map. For example, the temperature contour map shows warmth in red and coldness in gray purple, which highlights the contrast between cold and warm regions and their gradual relationship, and more vividly shows the regional change of temperature.