# 2. 10 Under what conditions are mixed cells a problem in raster data models? In what ways may the problem of mixed cells be addressed?

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• ## To effectively use GIS, it is important to understand:

• How coordinate systems are established for the surface of the Earth.
• How coordinates are measures on the Earth’s curved surface.
• How these coordinates are converted for use on flat maps

• ## Steps

• Define the size and shape of the Earth.
• Establish a datum – reference surface from which other points can be measured.
• Develop a spatial reference system:
• Origin
• Orientation of the axes
• Units of measure

• ## Three complicating factors:

• A flat map must distort geometry in some way.
• The irregular shape of the Earth.
• The imperfections of our measurements.

• ## Geographic coordinates in decimal degrees:

• Latitude is positive above the equator and negative below.
• Longitude is positive east of the Greenwich meridian and negative west of the Greenwich meridian.
• ## Two eras of North American datums:

• Must transform from one to the other.

• ## There are many different versions of the NAD83 datum.

• These should not be combined unless you are certain that the errors will be small.

• East Zone
• West Zone

• ## NAD 83 Maine State Plane (based on the TM projection)

• East Zone
• Central Zone
• West Zone

• ## Project

• Select a standard projection.
• Create a custom projection.
• Import a projection.

• ## When the representative fraction is relatively large we refer to the map as a large scale map.

• More detail
• Smaller area