GIS: Laying on the data

We can identify a location on a map using latitude and longitude.  Remember there are other ways to identify a location.  The US National Grid system is the Federal Geographic Data committee’s preferred method.  Amateur radio operators use the Maidenhead Locator System.  Technology today allows the coordinates to be converted from one system to another.  Just be very clear what coordinate system you are using and conform to accepted standards when writing out information.

Our world is physical.  All objects that you can touch have a location and take space.  You can interact with these, such as buildings, roads, mountains, water features, ground features and vegetation.

Objects also have non-physical properties, such as information about the object.  This information could be age, value, names, messages — anything that isn’t physical.  Inputs, outputs, relationships, risk are other non-physical features.

Think of people.  We all have physical characteristics that can be used to define us: height, weight, shape, color, strength, flexibility, etc.  We also have non-physical characteristics that can define us: intellect, emotions, spirituality, productivity, leadership, social networks, financial value, etc.

Both physical and non-physical characteristics of objects interact with other objects.  These can be textually explained but may be better understood with a graphical representation.  I could provide you a list of buildings and their floor space, but you may more quickly see the differences if we overlaid the plans of the buildings so you could roughly compare floor area and outer dimensions.

There is evidence that Cro-Magnon people drew animals and migration routes more than 15,000 years ago.  Dr. John Snow used mapping to study a cholera outbreak in London in 1854.  He drew a map of the neighborhood, drew points for the location of individual cases, and drew an X for the water pumps.  It was easily visible there was one pump in the middle of the outbreak.  Dr. Snow simply removed the handle from the pump and stopped the outbreak.

Original map made by John Snow in 1854. Cholera cases are highlighted in black.

Modern GIS is dependent on geocoding data and layers.  A complete database combined with solid analysis tools allows the leap from map making to true GIS analysis.  GIS is all about the relationships and space of data in the real world.

Layers were easier to describe when most people were familiar with acetate sheets (clear overhead transparency paper).  Imaging a stack of clear sheets where each sheet contained a different set of data.  The very bottom sheet was a base map.

A base map can be referred to as the common data, or the part of the map that you don’t need to create.  It can be the base geography, yet still up for debate.  A base map can be the geographic, demographic or topographic information that services as the common base.  ESRI lists the following as examples of base maps that may be selected: World Imagery, World Street Map, World Topographic Map, World Shaded Relief, World Physical Map, World Terrain Base, USA Topographic Maps, and Ocean Basemap.  In reality, the base map is just another layer that can be turned on or off.

Each layer contains a specific set of related data laid out by geographic coordinates.  Examples of different layers can be transportation systems, gas, electric, water & sewer, telecommunications, terrain, vegetation, buildings, hydrology and subsurface geology.

Layers will be turned on and off depending on what the end-user needs to see.  If I’m interested in the terrain and vegetation to predict wildland fire movements, then hide utilities since they won’t make a difference to the major motions.  A good GIS person will show major water features and highways as they will provide some fire breaks.

Just as the world exists in three dimensions, so can your layers.  Including the elevation (or height) in the geographic data will show the volume of something.  When modeling a hazardous plume, it can show if the plume is at the surface or above the population.  As Dale Loberger pointed out to me: flood analysis, plume dispersion or volumetric surface measures should be done in 3D.  Using technology to display a 3D model that allows you to view from many angles will hopefully reduce your urge to request a printed map when you really need an analysis tool.  Think of GIS as a visual interactive database.

Layers can also include time and historical elements: the fourth dimension (4D).  Historical data can be used to show the growth or shrink in a feature.  The wild land urban interface is an easy example.  Imagine that a community evaluated their risk and models a wild land fire near their community 10 years ago.  Today, their community has expanded yet the prediction has not been updated.  A map of three layers can be used to show what assumptions have changed.  The layers would be the community 10 years ago, the wild land fire prediction, and the community today.  Areas of rapid growth expansion will show.  Small roads 10 years ago may have been widened which may impact the direction and spread of the wildfire.

If earthquake research is my thing, then definitely keep the hydrology and subsurface information.  I’d like to see if the buildings in the community are built on solid footings, or if the ground will liquefy.  Once the major movement areas are identified, then add back the utilities to see where they cross higher risk areas.



GIS Continues: Vector and raster data types