There are many definitions but in its most simple form, GIS combines layers of information about entities in the real world (such as coral reefs, dive sites, ocean depth, or park locations) with hardware (computers), software (programs), and users (people like you) in order to understand spatial (geographic) relationships.  In most cases, GIS allows people to visualize and analyze data in the form of maps rather than complex databases- these maps can be powerful tools for making real world decisions such as where to place a restaurant, or how much law enforcement is needed to protect a certain area in a city.

NOTE: Environmental Systems Research Institute (ESRI), a major contributor to the GIS community, has prepared a complete PDF white paper on geography and GIS.  It is a valuable educational document for beginning GIS users. Geography Matters (319K)

Quick Tips for Online Mapping and GIS

The following are excerpts from the NOAA Coastal Services Center (CSC) Ocean Planning Information System (OPIS).  We believe these excerpts are critical for understanding the nature of spatial datasets (maps).  We recommend that all users unfamiliar with geospatial data read through this prior to viewing the maps.  We have made our own comments below each excerpt:

Map Projections

“Maps are flat representations of Earth's curved surface. Locations on Earth's three-dimensional surface are referenced using the geographic coordinate system (latitude and longitude). Mathematically transforming these three-dimensional coordinates onto a two-dimensional surface (paper or digital map) is called "projection". This process unavoidably distorts at least one the following properties: area, distance, shape, and direction (ESRI, 1994). Because there are an infinite number of map projections, a single "best" projection for any given application does not exist… The error caused by projecting data has been calculated and minimized where possible…”

 

NCORE Comment: Although every effort has been made to ensure that all datasets in the DNav (South Florida) are as accurate and correct as possible, project staff has no control over several external sources of error (cartographic interpretation, drafting errors, data conversions, precision, datum information, etc.).  All thematic maps have been re-projected to the Florida Geographic Data Library (FGDL) standard http://www.fgdl.org/fgdldocs/index.htm#proj . 

Scale

“Map Scale specifies the amount of reduction between the real world and the graphic representation on a map. It is usually expressed graphically, as a fraction (1/20,000), a ratio (1:20,000), or equivalence (1mm = 20m). Since map scale is most often used to describe paper map products, it is often assumed that the scale is fixed, and cannot change. However, a map in a GIS can be shrunk or enlarged on the screen by zooming in or out. This implies that geographic data in a GIS does not really have a true "map scale".

When scale is used to describe digital data, it is often referring to the scale of the source data or the scale at which the digital data looks "right". As a result, this display scale influences the amount of detail that can be shown (Foote et. al. 1995). Digital data viewed at inappropriate display scales within a GIS can be misleading…”

  

NCORE Comment: Always refer to the metadata to see what the most appropriate scales are for a particular dataset. 

Map Resolution Accuracy

“Map resolution refers to the accuracy of the location and shape of a map feature shown at a given scale. In general, as map scale increases (e.g. 1:100k to 1:50k to 1:20k), so do map resolution and accuracy. However, accuracy is also affected by the quality of source data used to map a feature (ESRI, 1994).  Features on large-scale maps more closely represent the real world because the amount of reduction (from real world to map) is less. As the level of detail of a paper map increases for a given area of earth, the size of the paper map required to cover the same area also increases. Similarly, as digital map resolutions become more detailed and accurate, file sizes increase because more information is now represented for the same area. Producing high-resolution digital data for a large area of the earth results in very large files. Larger file sizes result in larger amounts of data to organize and manage. A fundamental challenge for a regional GIS is to provide data for a very large area at a scale detailed enough to enable resource managers to make sound decisions, yet not so detailed that the amount of data becomes too large to manage…”

 

NCORE Comment: The DNav (South Florida) Project covers a fairly sizeable land area. The resolution and accuracy of individual data layers are affected correspondingly.  Always refer to the metadata to gather information regarding the resolution and accuracy of a particular dataset.

Multiple Data Sources and Problems with Undocumented Data

Because data for the DNav (South Florida) were acquired from multiple sources in a variety of formats with varying accuracy standards and processing techniques (refer to the metadata records for specific information on data), some inconsistencies were encountered. Hence, users should be aware that data deficiencies and, in some cases, gaps exist. DNav (South Florida) staff are working on these inconsistencies and will post modifications on an ongoing basis.   

References

(ESRI) Environmental Systems Research Institute, Inc. (1994) Map Projections, Georeferencing spatial data.

 

Kenneth E. Foote and Donald J. Huebner, The Geographer's Craft Project, Department of Geography, University of Texas at Austin. All commercial rights reserved. Copyright 1995 Kenneth E. Foote and Donald J. Huebner.