Maps use projections to transfer real world information about the Earth’s surface to a flat surface. This information always results in some form of distortion. Area, shape, direction, and distance are misrepresented in the process. This distortion is unavoidable when transferring 3-D information onto a flat surface.
Misaligned or improperly referenced spatial data can be fixed by checking the layer’s geographic coordinate system and datum. ArcGIS allows the user to check and change coordinate reference information. The geographic coordinate system and datum must be appropriate choices: a North American GCS and datum should be used for studying areas in North America. They should also be the same systems being used in the data frame. The geographic coordinate system and datum can be checked in its properties tab in the Table of Contents. It can be changed in the Catalog tab, under the XY Coordinate System tab.
To change a layer’s projection, use the Toolbox icon. Under Data Management, choose Projections and Transformations, then Define Projection. Enter the layer’s name, then the desired projection reference system. The projection of layers must be aligned in order to perform accurate calculations and observations of maps created in ArcGIS.
Landsat data is a valuable toll for studying the Earth’s surface because it uses a multi-spectral sensor array. It is a passive remote sensor, so it picks up varying wave lengths of energy emitted or bounced off from the Earth. Light, and several infra-red wavelengths are gathered. This allows the satellite to see vegetation, water bodies, agriculture, land-use, and pollution on the Earth’s surface. Landsat takes millions of pictures, so two pictures taken of the same place at different times can be compared to study differences in the landscape.
