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and the coordinate system is unknown , then the user may or may not be able to convert the data to the correct units. It is best to inquire about the coordinate system before receiving data . ArcGIS software includes tools to convert between coordinate systems . Because coordinate conversion can be cumbersome and time consuming , requesting or providing all map projection information with exchange files is crucial . This information should be provided as a standard in all metodata files that accompany Indian River County data , or are requested from outside sources ( see Metadato section below) . The existing standard utilized by Indian River County is NAD 83 , Florida East, U .S . Feet ( this is the same as the recommended coordinate system presented below; see " Recommended Coordinate for Indian River County" below) . Datums Another consideration along with the coordinate system is datum . There are two horizontal datums used almost exclusively in North America . These are the North American Datum of 1927 ( NAD27) and the North American Datum of 1983 ( NAD83) . It is recommended that cadastral maps in Florida be based upon state plane coordinates adjusted to the 1983 horizontal datum . Other projections and coordinate systems may be used to satisfy special requirements . The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent the shape of the Earth . The origin of this datum is a point on the Earth referred to as Meades Ranch in Kansas . Many NAD27 control points were calculated from observations taken in the 1800s . " These calculations were done manually and in sections over many years . As a result of this process , errors varied from station to station . Many technological advances in surveying and geodesy since the establishment of NAD27 - electronic theodolites, GPS satellites , Very Long Baseline Interferometry , and Doppler systems - revealed weaknesses in the existing network of control points . Differences have become particularly noticeable when linking existing control with newly established surveys . The establishment of a new datum allowed for a single datum to cover consistently North America and surrounding areas . The North American Datum of 1983 is based on both Earth and satellite observations , using the GRS80 spheroid . The origin for this datum is the Earth ' s center of mass . This affects the surface location of all latitude-longitude values enough to cause locations of previous control points in North America to shift , sometimes as much as 500 feet. A ten-year multi- national effort tied together a network of control points for the United States , Canada , Mexico, Greenland , Central America , and the Caribbean . NAD27 and NAD83 should not be used in conjunction with one another. If any NAD27 derived data exists then convert it to NAD83 . Tools within GIS software make this a relatively easy process . Indian River County does not currently have a standard . Technology Tip - If two GIS layers used in conjunction with each other are consistently offset by approximately 8- 10 feet, it is probable that there is a " datum mismatch . It is likely that one layer is utilizing NAD27 datum and the other NAD83 datum . The NAD27 layer should be converted to NAD83 . 6-4 GIS Needs Assessment and Implementation Plan Chapter 6 - Data Standards and Transfer