Introduction to GIS

What is GIS?

• Geographic Information System

• Geographic implies of or pertaining to the surface of the earth

• Information implies knowledge of or collection of some form of data

• and finally, a system implies some form of organization, arrangement, etc.. Perhaps a framework?

........ A computer system for capturing, managing, integrating, manipulating, analyzing and displaying data which is spatially referenced to the Earth." (R McDonnell & K Kemp. 1995. International GIS Dictionary. Cambridge: GeoInformation International)

GIS Components

• Overview Components:– Data

– Maps/Views/Layouts

– Spatial Analysis

• Physical Components– software

– hardware

– data

– users

– need/application

GIS Software

• Many types• Around for > 30 years• Only really prevalent for ~20 yrs?• We will be using ArcView by ESRI• Also ArcInfo by ESRI• GIS software is loosely defined• Some more applicable to certain tasks - ERDAS

for example.

Hardware

• Software may dictate hardware in some cases - AWIPS!

• Generally:

• computer

• database

• printers and plotters, etc....

Data

• From many sources

• field collected

• spatial in nature

• point, line, polygons, grids

• METADATA!!!!!!!

Users

• Very wide range

• some users, some doers, etc...

• some want pictures

• some want answers

• some want to do analysis

• know your audience!

Applications

• What is the desired final product?

• Will the tasks be repeated over and over?

• Or is this a one time application?

What does a GIS do?

• Input data

• Manage data

• Manipulate data

• Perform analyses

• Produce output - maps, charts, a single number?

Let’s jump right to Arcview

• Organizes your data into a PROJECT

• A PROJECT contains:

• views, tables, charts, layouts, and scripts.

• Each of the above components also contians so-called components.

• Notice that I did not mention DATA!!!!!

Views

• Display themes or geographic data

• A view has an area for displaying the theme and an area for the “table of contents” - or what views are presently loaded.

Tables

• Display tabular data or the descriptive attributes of the elements in the data set.

Charts

• Display data graphically

• Different than the map type displays

Layouts

• The whole shebang?!?!?!

• Put it all together

Scripts

• Small computer programs

• Allow repeatability.....

• AVENUE

The Watershed

• A watershed is an area of land that drains to a single outlet and is separated from other watersheds by a divide.

• Hydrologic analysis and synthesis focus on the watershed.

The Watershed

• Every watershed has a drainage area.

• Related terms: drainage basin, sub-basin, sub-area.

Defining a Watershed

• Defining a watershed is generally referred to as delineating the watershed.

• The process involves determining that area within which water would drain to a common point.

• It is often easier to visualize the concept by pretending the ground surface is impermeable like cement.

Defining a Watershed

Contours are lines of constant elevation.

Contours “point” or “curve” uphill at stream crossings.

Contours (generally) have constant spacing.

Defining a Watershed

• At right, the watershed has been delineated, using the contours, for the indicated watershed outlet.

• The streams/channel sections have also been highlighted within the watershed.

Defining a Watershed

• The most common form of mapping used for delineation are the USGS topographical maps.

• The most common map scale is the 7.5 minute 1:24,000 scale.

Defining a Watershed• The use of electronic or digital

data and mapping has become rather common place in the field of hydrology.

• GIS or Geographical Information Systems are used to manage, manipulate, and analyze digital data.

• One of the most common GIS data sets is a Digital Elevation Models or DEM’s.

Defining a Watershed

This section is dedicated to providing a basic understanding of the methods used by a Geographical Information System (GIS) programs to delineate a watershed.

Defining a Watershed

A GIS uses electronic data sets. The electronic data sets or coverages represent some property of the earth’s surface. One of the most common data sets is a Digital Elevation Model or DEM.

Defining a Watershed

A DEM is used to represent the elevations of a section or area of land. The DEM is generally a gridded or raster data set. A raster data set means a series of rows and columns with each grid cell representing a property such as elevation.

Defining a WatershedShown below (in 3-D) is a simple DEM, which is comprised of 12 rows and 12 columns.

02

46

810

row0

24

68

10col

725

750

775

800

825

jen1_ele

740 760 780 800 820

jen1_ele

Defining a WatershedIn a plan view, the same DEM looks like :

0 2 4 6 8 10

0

2

4

6

8

10

col

row

740 760 780 800 820

jen1_ele

Defining a WatershedThe same DEM shown in an “interpolated” format:

0 2 4 6 8 10

0

2

4

6

8

10

col

row

740 760 780 800 820

jen1_ele

Defining a WatershedThe actual values of the grid cells in the rows and columns are illustrated in the table below:

778 765 750 740 747 759 765 766 769 776 786 795770 758 745 737 741 751 753 761 777 789 802 814777 763 747 736 735 743 750 767 787 806 820 832786 767 750 737 733 739 752 769 785 797 808 822791 773 756 741 733 733 744 759 772 779 789 806799 782 763 750 737 733 733 745 757 767 782 801802 788 771 761 751 736 733 738 751 764 779 798799 790 780 772 762 746 733 737 754 770 784 794811 799 787 771 757 741 728 730 745 765 779 783823 807 790 774 762 748 733 725 733 750 764 763830 814 801 787 776 761 743 728 725 737 748 751822 818 811 801 791 776 757 739 726 725 735 751

Defining a WatershedIn the DEM, each grid cell must be assigned a “flow direction”. It is the direction that the water would run or flow. One of the most common methods is the D8 algorithm. The D8 algorithm looks at 3x3 windows and assigns a flow direction to the cell in the center by considering the direction of the largest drop in elevation.

Defining a WatershedConsider the 3x3 group of cells below. The center cell flows or points in the direction indicated because that is the direction of the greatest elevation drop.

750 737 733761 751 736772 762 746

Defining a WatershedThe flow direction is then assigned and recorded. In this example, the flow directions are simply 0 to 7 as shown below:

6 7 05 X 14 3 2

Defining a WatershedEach grid cell is assigned a flow direction. The resulting image of flow directions for the small 12x12 example is illustrated below:

0 2 4 6 8 10

0

2

4

6

8

10

col

row

0 1 2 3 4 5 6 7

jen1_asp

Defining a Watershed

Note that each grid cell flows into another grid cell and so forth. One could tally or keep track of the total number of grid cells that flow into each “downstream” grid cell. This is generally know as a flow accumulation data set. The flow accumulation data set for the 12x12 example is illustrated on the next page:

Defining a Watershed

0 2 4 6 8 10

0

2

4

6

8

10

col

row

0 10 20 30 40 50 60 70 80 90

jen1_cou

Flow accumulation data set