الخرائط الموضوعية خرائط تركز بياناتها على غرض بعينه تعني :إظهار البيانات الوصفية في أسلوب رسومي بتغيير مظهر المعالم مما يجعل المعلومات أكثر وضوحاً
مثلاً : - استخدام دوائر أكبر لترميز المدن ذات عدد السكان الأكبر - استخدام خطوط عريضة لترميز الطرق ذات الكثافة المرورية العالية
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يشوه مسقط ماركيتور كل من الشكل والمسافة والمساحة بعيداً عن خط الاستواء ولكنه يحتفظ بالاتجاه صحيحاً.
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Datums and datum conversion
A horizontal datum is a reference frame used to locate features on the earth’s surface. It is defined by an ellipsoid and that ellipsoid’s position relative to the earth. There are two types of datums: earth-centered and local. An earth-centered datum has its origin placed at the earth’s currently known center of mass and is more accurate overall. A local datum is aligned so that it closely corresponds to the earth’s surface for a particular area and can be more accurate for that particular area. Within both of the basic types of datums, you can have several global and local datums. Because datums establish reference points to measure surface locations, they also enable us to calculate planar coordinate values when applying a projection to a particular area.
Ellipsoid
The earth is often treated as a sphere to make mathematical calculations easier; however, its shape is actually an ellipsoid. Rotating an ellipse about an axis forms an ellipsoid. An ellipsoid is like a flattened circle with radius lengths along its major and minor axes of length a and b, respectively. The diagram shows that the ellipsoid is symmetric when divided at the equator (i.e., the southern hemisphere and the northern hemisphere are identical in shape). This is not strictly correct, because the earth is slightly pear-shaped; however, the difference in shape between the hemispheres is very slight.