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GIS Data Basics

This section deals with the types of data used in GIS.

 

In GIS, everything in the real world is turned into a feature on a map, and each of those features have data that can be used to depict or analyze them. Data in GIS is often called a "layer" (other terms can include "theme", "coverage" or "shape").

 

Places and Data 


One way to imagine this is to see GIS as the marriage of a place and a spreadsheet. For example, a GIS can show where each individual land trust holding is located (the places), and also have information (the spreadsheet data) about each site, such as its size, use, date acquired, etc.

The place features (in this case land parcels) are linked to the data by a common ID number (for land holdings, typically the parcel's official tax number). This allows GIS maps to be dynamic, as the data is displayed to meet different purposes ("show the value of all our holdings, grouped in 4 dollar size ranges", or "show all owners who have two or more easements with us").

In GIS, all data is defined as either: 

  • Points (one person’s address location)
  • Lines (a street or stream)
  • Polygons (a parcel or a census tract), or
  • Rasters (digital photos or other data made up of pixels or grids) - see below for more information

These "layers" form the core of map-based GIS - by stacking them, you can select between them, easily creating many different views of a place. Layers in GIS typically contain just one type of data (e.g., not points and lines together, but points and lines separately). However, in some cases GIS data is stored in a "geodatabase", a more complex system that allows many different types of data to be managed as a system.

 

Creating Data


Data is created in a number of ways.  It can be digitized (each point of a feature drawn by hand on a GIS screen or on a special digitizing table), scanned (a picture is made of the map or air photo then given latitude/longitude reference points, often called "rubbersheeting"), or it can be geocoded (mostly for points, where each point is assigned a place on a street grid based on its address).  In some cases, a paper map can be scanned and auto-digitized by GIS software.

For land trusts, the most likely way of creating property holdings, trails or other data is from a paper "manuscript" map (such as a USGS quad). The object(s) are drawn exactly on the map and an operator then makes (or traces) the same shape as a digital object using GIS software. Each created feature (e.g., a holding, a building) is then given a unique identification number so that more data about the feature (the "spreadsheet") can be linked with the feature itself. 

More About Raster Data:    "Raster" is a fancy name for data that is made up of cells. For example, if you examine a digital photo closely by zooming in very far, you'll see that a smooth looking image is actually composed of a series of small squares or pixels.  Digital photos are often used by land trusts as just backdrops, with parcel or other data overlaid on them.

However, GIS can also use raster data as a  “grid” (a series of small squares) - by assigning the dominant use value to each square. this raster data can then be analyzed.

For example, using a special type of aerial photo showing vegetation, you can make a grid of squares that show the primary vegetation type for each cell. The cells might be 10x10 meters or as big as 100 meters square.  You can then group the cells by vegetation type and assign additional data values to each cell based, for example, on its proximity to riparian corridors. 

 

Issues With GIS Data


GIS data is widely available and often is free or very modest in price.  Working with multiple sets of data, however, can at times be very frustrating. Here are the most common issues:

Fitting together:  When you zoom in on a place, you may sometimes find that roads and parcel lines don't match up, or streams are tens (or more!) of feet from where an aerial photo says they are.  This is often because one set of data was created using better base maps or with more care for accuracy.  There is no easy fix for this, short of editing the less accurate data to correspond better with the other.  Generally, land trusts should use USGS quads or their digital equivalent for creating data; air photos that are sufficiently detailed are also a good base. The best base information is parcel boundaries that have been created by local governments in GIS format. (link to parcel boundary discussion).

Differing resolutions:  When using raster data (aerial images or pictures of maps such as USGS quads, which are made up of very small squares, or pixels), the resolution of that data is important to know. For instance, if you zoom in very close to get an exact property boundary, you may find that instead of a clearer view, all you see are blocky pixels. In that case, you've gone in farther than your data will support.

Projections:   A key issue with data is its “projection” – how it is placed in the geographic world.  Going from a flat map to a spherical, actual world requires translating data points (using coordinates) from one framework to the other.

With names like State Plane, UTM, Albers and many others, projections can be quite thorny for novice GIS users (compounded by a related factor, called a “datum” which defines the surface of the earth as of a certain date).

Fortunately, recent changes in GIS software make this somewhat easier, but it can still be troublesome when you add data to your GIS view and find that the road that should be next to a particular feature is actually hundreds or thousand miles away, due to different projections.  

The solution to projection issues is generally: create a file that defines the data's original projection; or, re-project the data to a new file with a more common projection. For both of these tasks you will need to find guidance in your GIS software, under "projections".

LEARN MORE about projections...

 

Data Cost and Licensing


Much GIS data is free or available at minimal cost. However, commercial vendors do provide some premium or speciality data sets that can be very useful, and some governmental agencies attempt to charge for data. There are debates over whether governmental bodies can charge more than cost of reproduction (for federal agencies, see Dept. of Justice's Freedom of Information Act web site; for state and local agencies, see if you have a state public records act).

In additional various data providers may have license restrictions that you, as a user, must follow. It is important to know these before you invest in paid-for data in particular.

NEXT – GIS Software Basics >>>

 

MORE INFORMATION

Primer on USGS GIS Data

GIS.com Overview of GIS Data

 
© Land Trust GIS 2006