GIS Basics

Manipulating Geodata

If you’re a planner, a student, or an engineer, you will use geographic data. Sometimes you’ll require special skills to perform a task but more often than not you will need to simply convert or display the data. These tools (most of them online) will help you.

Shapefile (SHP)

This native file format to ESRI‘s ArcGIS is the most commonly used environment for geodatabase files, and allowsfor the combined storage and transfer of geometric (raster or vector), locational (coordinate), and attribute (descriptive) data in one container. This container will most commonly be shared as a zip file containing all of the files needed to properly display the shapefile in your GIS.

From Wikipedia –

Mandatory files :

  • .shp — shape format; the feature geometry itself
  • .shx — shape index format; a positional index of the feature geometry to allow seeking forwards and backwards quickly
  • .dbf — attribute format; columnar attributes for each shape, in dBase IV format

Optional files :

  • .prj — projection format; the coordinate system and projection information, a plain text file describing the projection using well-known text format
  • .sbn and .sbx — a spatial index of the features
  • .fbn and .fbx — a spatial index of the features for shapefiles that are read-only
  • .ain and .aih — an attribute index of the active fields in a table
  • .ixs — a geocoding index for read-write shapefiles
  • .mxs — a geocoding index for read-write shapefiles (ODB format)
  • .atx — an attribute index for the .dbf file in the form of shapefile.columnname.atx (ArcGIS 8 and later)
  • .shp.xmlgeospatial metadata in XML format, such as ISO 19115 or other XML schema
  • .cpg — used to specify the code page (only for .dbf) for identifying the character encoding to be used

 

Keyhole Markup Language (KML)

The native file format for Google’s 2D and 3D display of geographic data. This format is a common export of many hand-held GPS units and mobile apps. It may take some maneuveringto get your data in and out of this format if you want to use it in a GIS, but it is rapidly becoming a standard for data transport. Similar to the shapefile, the KML is often packaged as a zip file (KMZ) where you’ll house your photos and other associated data with the KML, and unpack it to display individual objects when embedding a KML viewer onto your site.

GeoCoding

One of the most essential tasks in web mapping is to accurately identify the location of your information. Essentially you are just assuring that the lat/long of any given point (or series of lat/longs in lines and polygons) is accurate so it will display where it is supposed to in your map. There are a number of different techniques and algorithms for accomplishing this goal, as many projects involve geocoding hundreds and thousands of addresses or locations in one set. Certainly, if you only have a handful of places you can manually create your lat/long column, but by far the industry leader for large batches and even a simple verification of a small set is Batch Geocode.

  • BatchGeocode.com – Forward and reverse geocode addresses and geographic coordinates. Input tabular data copied directly from your spreadsheet (in Microsoft Excel, OpenOffice.org Calc, or Google Docs). Download results as a Google Earth and Maps-compatible KML file or copy back into your spreadsheet. The web application even allows for simple grouping (to which it will apply unique map icons).

Geographic Positioning System (GPS)

There are entire courses taught on the use of GPS for acquiring data, and so this summary will hardly do it justice. Essentially, if there are things you want to include in your map that you can’t find the geographic location through other means, the best way is to just go out into the field and collect it yourself. GPS takes data from satellites – also where mufti-spectral images of the earth for land assessment and weather monitoring come from – and stores locations along with the attribute data you enter into the associated “data dictionary.” The complexity of this topic arises from the wide variety of geometric accuracy that can be attained from using different collection devices. Professional Field Survey technicians use equipment that can come with in a centimeter of an accurate reading, while most hand held devices range anywhere from 5-30 meters. Of course, professional Field Surveyors are licensed and use very expensive equipment, and most commonly they are contracted by Civil Engineers and Property Lawyers who really need to know exact details. For the majority of uses though, gathering data for general description of locations can be satisfied with lower accuracy and lower cost devices. Especially as it is always good practice to corroborate your field collections in a GIS before publishing them, most errors can be adjusted for post-capture.

  • GPS Visualizer – “A free, easy-to-use online utility that creates maps and profiles from GPS data (tracks and waypoints, including GPX files), street addresses, or simple coordinates. Use it to see where you’ve been, plan where you’re going, or visualize geographic data (business locations, scientific observations, events, customers, real estate, geotagged photos, ‘GPS drawing,’ etc.).”

Geographic Information Systems (GIS)

Alternatives to ESRI ArcGIS

Working with the Data

Both your spatial data and your attribute data will require certain steps and processes to produce the desired results of a map that accurately conveys a location and a concept. Cleaning your data so that it works within your GIS environment, and eventually your web environment, involves normalizing to ensure data conformity. There are a variety of different tools on the market for data cleaning that range from simple Excel Spreadsheets to multi-level databases. OpenRefine is a great tool for quick and reliable edits, and has quite a bit more functionality than simple spreadsheets. For your spatial data, take special care of the Geographic Projection (algorithm used to convert 3D data to 2D representation), and be observant of the project that data you obtained comes in. Then learn which coordinate reference systems and projections are common for that type of data and that location.

 

Updated and Cross Posted from Steven Can Plan

Margaret Spyker

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