GIS in Water Resources

Review for Midterm Exam

Data Models

• A geographic data model is a structure for organizing geospatial data so that it can be easily stored and retrieved.

Geographic coordinates

Tabular attributes

Raster and Vector Data

PointPoint

LineLine

PolygonPolygon

VectorVector RasterRaster

Raster data are described by a cell grid, one value per cell

Zone of cells

ArcGIS Geodatabase

Geodatabase

Feature Dataset

Feature Class

Geometric Network

Object Class

Relationship

Workspace

Geodatabase and Feature Dataset

A geodatabase is a relational database

that stores geographic information.

A feature dataset is a collection of

feature classes that share the same

spatial reference frame.

Feature Class

• A feature class is a collection of geographic objects in tabular format that have the same behavior and the same attributes.

Feature Class = Object class + spatial coordinates

Object Class• An object class is a collection of

objects in tabular format that have the same behavior and the same attributes.

An object class is a table that has a unique identifier (ObjectID)for each record

RelationshipRelationship between spatial and non-spatial objects

Water quality data(non-spatial)

Measurement station(spatial)

National Hydro Data ProgramsNational Elevation Dataset

(NED)National Hydrography Dataset

(NHD)

Watershed Boundary DatasetNED-Hydrology

http://www.ftw.nrcs.usda.gov/stat_data.html

1:250,000 Scale Soil Information

National Land Cover Datasethttp://landcover.usgs.gov/nationallandcover.html

http://seamless.usgs.gov/Get the data:

National Water Information System

Web access to USGS water resources data in real time

http://waterdata.usgs.gov/usa/nwis/

Flow

Time

Time Series

Hydrography

Hydro Network

Channel System

Drainage System

Arc Hydro ComponentsGIS provides for synthesis of geospatial data with different formats

Geodesy, Map Projections and Coordinate Systems

• Geodesy - the shape of the earth and definition of earth datums

• Map Projection - the transformation of a curved earth to a flat map

• Coordinate systems - (x,y) coordinate systems for map data

Latitude and Longitude in North America

90 W120 W 60 W

30 N

0 N

60 N

Austin: (30°N, 98°W)

Logan:(42°N, 112°W)

Length on Meridians and Parallels

0 N

30 N

Re

Re

RR

A

BC

(Lat, Long) = (, )

Length on a Meridian:AB = Re (same for all latitudes)

Length on a Parallel:CD = R Re Cos(varies with latitude)

D

Example 1: What is the length of a 1º increment along on a meridian and on a parallel at 30N, 90W?Radius of the earth = 6370 km.

Solution: • A 1º angle has first to be converted to radians radians = 180 º, so 1º = /180 = 3.1416/180 = 0.0175 radians

• For the meridian, L = Re km

• For the parallel, L = Re CosCoskm• Parallels converge as poles are approached

Example 2: What is the size of a 1 arc-second DEM cell when projected to (x,y) coordinates at 30º N?Radius of the earth = 6370 km = 6,370,000m = 6.37 x 106 m

Solution: • A 1” angle has first to be converted to radians radians = 180 º, so 1” = 1/3600 º = (1/3600)/180 radians = 4.848 x 10-6 radians

• For the left and right sides, L = Re 6.37 x 106 * 4.848 x 10-6 =30.88m

• For the top and bottom sides, L = Re Cos= 6.37 x 106 * 4.848 x 10-6 * Cos 30º = 30.88 x 0.8660 = 26.75m

• Left and right sides of cell converge as poles are approached

Horizontal Earth Datums• An earth datum is defined by an ellipse and

an axis of rotation• NAD27 (North American Datum of 1927)

uses the Clarke (1866) ellipsoid on a non geocentric axis of rotation

• NAD83 (NAD,1983) uses the GRS80 ellipsoid on a geocentric axis of rotation

• WGS84 (World Geodetic System of 1984) uses GRS80, almost the same as NAD83

Vertical Earth Datums

• A vertical datum defines elevation, z

• NGVD29 (National Geodetic Vertical Datum of 1929)

• NAVD88 (North American Vertical Datum of 1988)

• takes into account a map of gravity anomalies between the ellipsoid and the geoid

Coordinate System

(o,o)(xo,yo)

X

Y

Origin

A planar coordinate system is defined by a pairof orthogonal (x,y) axes drawn through an origin

Universal Transverse Mercator

• Uses the Transverse Mercator projection

• Each zone has a Central Meridian (o), zones are 6° wide, and go from pole to pole

• 60 zones cover the earth from East to West

• Reference Latitude (o), is the equator

• (Xshift, Yshift) = (xo,yo) = (500000, 0) in the Northern Hemisphere, units are meters

UTM Zone 14

Equator-120° -90 ° -60 °

-102° -96°

-99°

Origin

6°

ArcInfo 9 Spatial Reference Frames

• Defined for a feature dataset in ArcCatalog

• Coordinate System– Projected

– Geographic

• X/Y Domain• Z Domain• M Domain

X/Y Domain

(Min X, Min Y)

(Max X, Max Y)

Maximum resolution of a point = Map Units / Precisione.g. map units = meters, precision = 1000, thenmaximum resolution = 1 meter/1000 = 1 mm on the ground

Long integer max value of 231 = 2,147,483,645

Four Points

One degree box and its four lines

Geographic Coordinates

One Degree Box in USGS Albers Projection

USGS Albers Projection

Area Calculation in USGS Albers

Area = 9130.6 km2111.

79 k

m

111.

79 k

m

82.26 km

81.09 km

82.26 + 81.09

2x 111.79 = 9130.5 km2

North American Albers Projection

Same projection method as USGS Albers but different parameters

Area Calculation in North American Albers

Area = 9130.6 km2118.

17 k

m

118.

17 k

m

77.89 km

76.64 km

77.89 + 76.64

2X 118.17 = 9130.4

Take home message: Lengths of lines change but area is constant in Albers

x

dx)y,x(f)y(f

x

y

f(x,y)

Two fundamental ways of representing geography are discrete objects and fields.

The discrete object view represents the real world as objects with well defined boundaries in empty space.

The field view represents the real world as a finite number of variables, each one defined at each possible position.

(x1,y1)

Points Lines Polygons

Continuous surface

Vector and Raster Representation of Spatial Fields

Vector Raster

Numerical representation of a spatial surface (field)

Grid

TIN Contour and flowline

Grid Datasets• Cellular-based data structure composed of

square cells of equal size arranged in rows and columns.

• The grid cell size and extent (number of rows and columns), as well as the value at each cell have to be stored as part of the grid definition.

Number of columns

Num

ber

of

row

s

Cell size

Raster Sampling

from Michael F. Goodchild. (1997) Rasters, NCGIA Core Curriculum in GIScience, http://www.ncgia.ucsb.edu/giscc/units/u055/u055.html, posted October 23, 1997

From: Blöschl, G., (1996), Scale and Scaling in Hydrology, Habilitationsschrift, Weiner Mitteilungen Wasser Abwasser Gewasser, Wien, 346 p.

Extent

Spacing

The scale triplet

Support

Spatial Generalization

Central point ruleLargest share rule

Raster calculation – some subtleties

Analysis extent

+

=

Analysis cell size

Analysis mask

Resampling or interpolation (and reprojection) of inputs to target extent, cell size, and projection within region defined by analysis mask

InterpolationEstimate values between known values.

A set of spatial analyst functions that predict values for a surface from a limited number of sample points creating a continuous raster.

Apparent improvement in resolution may not be justified

Topographic Slope

• Defined or represented by one of the following– Surface derivative z

– Vector with x and y components

– Vector with magnitude (slope) and direction (aspect)

Hydrologic processes are different on hillslopes and in channels. It is important to recognize this and account for this in models.

Drainage area can be concentrated or dispersed (specific catchment area) representing concentrated or dispersed flow.

Drainage Density Dd = L/AEPA Reach Files 100 grid cell threshold 1000 grid cell threshold

Network Definition

• A network is a set of edges and junctions that

are topologically connected to each other.

Edges and Junctions

• Simple feature classes: points and lines• Network feature classes: junctions and edges• Edges can be

– Simple: one attribute record for a single edge

– Complex: one attribute record for several edges in a linear sequence

• A single edge cannot be branched

No!!

Polylines and Edges

Junctions

• Junctions exist at all points where edges join– If necessary they are added during network

building (generic junctions)

• Junctions can be placed on the interior of an edge e.g. stream gage

• Any number of point feature classes can be built into junctions on a single network

Connectivity Table

J124

J125

J123J126

E1 E3

E2J123 J124, E1

J124 J123, E1 J125, E2 J126, E3

J125 J124, E2

J126 J124, E3

Junction Adjacent Junction and Edge

This is the “Logical Network”

p. 132 of Modeling our World

Flow to a sink

Network Tracing on the Guadalupe Basin

Linear Referencing

Where are we on a line?

Addressing

Arc Hydro Framework with Time Series

Spatial relationship classes

Temporal classes and relationships

Geometric network

Space-Time CubeTSDateTime

TSTypeID

TSValue

FeatureID

Time

Space

Variable

Data Value

MonitoringPointHasTimeSeries Relationship

TSTypeHasTimeSeries

Arc Hydro TSType Table

TypeIndex

VariableName

TypeOf

TimeSeries

Info

Regular or

Irregular

Unitsof

measure

Timeinterval

Recordedor

Generated

Arc Hydro has 6 Time Series DataTypes1. Instantaneous2. Cumulative3. Incremental4. Average5. Maximum6. Minimum

Tracking Analyst Display

DEM Based Watershed and Stream Network Delineation Steps

• DEM Reconditioning/Burning in Streams• Fill Sinks• Eight direction pour point model to evaluate flow

directions• Flow accumulation• Threshold stream network definition• Stream segmentation• Watershed delineation• Raster to vector conversion of streams and

watersheds

+ =

Take a mapped stream network and a DEM Make a grid of the streams Raise the off-stream DEM cells by an arbitrary elevation increment Produces "burned in" DEM streams = mapped streams

“Burning In” the Streams

Synthesis of Raster and Vector data

AGREE Elevation Grid Modification Methodology

ORIGINAL ELEVATIONMODIFIED ELEVATION

KNOWN STREAM LOCATIONAND STREAM DELINEATEDFROM MODIFIED ELEVATION

GRID CELL SIZESECTION A-A

STREAM DELINEATEDFROM ORIGINAL ELEVATION

ELEVATIONRESOLUTION

GRIDCELL SIZE

PLAN

AA

Filling in the Pits

• DEM creation results in artificial pits in the landscape

• A pit is a set of one or more cells which has no downstream cells around it

• Unless these pits are filled they become sinks and isolate portions of the watershed

• Pit filling is first thing done with a DEM

67 56 49

52 48 37

58 55 22

30

67 56 49

52 48 37

58 55 22

30

45.0230

4867

50.0

30

5267

Slope:

Hydrologic Slope - Direction of Steepest Descent

32

16

8

64

4

128

1

2

Eight Direction Pour Point Model

Water flows in the direction of steepest descent

Flow Direction Grid

32

16

8

64

4

128

1

2

Cell to Cell Grid NetworkThrough the Landscape

Stream cell

Contributing Area Grid

1 1 111

1

1

1

1

1

1

1

1

14 3 3

12 2

2

3 2

16

625

1 1 11 1

1

1

1

1

1

1

1

1

1

4 3 3

12 2

2

23

16

256

Drainage area threshold > 5 Cells

Delineation of Streams and Watersheds on a DEM

Watershed and Drainage Paths Delineated from 30m DEM

Automated method is more consistent than hand delineation

55

11

1

3

22

334 4 4

4 45

5

666

Stream Segments in a Cell Network

Same Cell Value

Subwatersheds for Stream Segments

Vectorized Streams Linked Using Grid Code to Cell Equivalents

VectorStreams

GridStreams

• For every stream segment, there is a corresponding catchment

• Catchments are a tessellation of the landscape through a set of physical rules

Delineated Catchments and Stream Networks

Raster Zones and Vector Polygons

Catchment GridID

Vector Polygons

DEM GridCode

Raster Zones

3

4

5

One to one connection

Watershed

• A watershed is the area draining to any point on the stream network

• A new kind of connectivity: Area flows to a point on a line

Connecting Drainage Areas to the Network

Area goes topoint on line

HydroID – a unique identifier of all Arc Hydro features

HydroIDs of Drainage Points HydroIDs of Catchments