Data Model

Conceptual Model – a set of concepts that describe a subject and allow reasoning about it

Mathematical Model – a conceptual model expressed in symbols and equations

Data Model – a conceptual model expressed in a data structure (e.g. ascii files, Excel tables,

…..)Geographic Data Model – a conceptual model for describing and reasoning about the world

expressed in a GIS database

Geographic coordinates

Tabular attributes

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

Raster and Vector Data

PointPoint

LineLine

PolygonPolygon

VectorVector RasterRaster

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

Zone of cells

Themes or Data Layers

Vector data: point, line or polygon features

How each of these features could be represented using vector or raster?

ArcGIS Geodatabase (what is in a geodatabase)

GeodatabaseFeature DatasetFeature Class

Geometric Network

Object ClassRelationship

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 (do not have a shape).

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)

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

Spatial Reference = Datum + Projection + Coordinate system

Types of Coordinate Systems• (1) Global Cartesian coordinates (x,y,z)

for the whole earth• (2) Geographic coordinates (, z) • (3) Projected coordinates (x, y, z) on a

local area of the earth’s surface• The z-coordinate in (1) and (3) is

defined geometrically; in (2) the z-coordinate is defined gravitationally

Global Cartesian Coordinates (x,y,z)

O

X

Z

Y

GreenwichMeridian

Equator

•

Geographic Coordinates (, z)

• Latitude () and Longitude () defined using an ellipsoid, an ellipse rotated about an axis

• Elevation (z) defined using geoid, a surface of constant gravitational potential

• Earth datums define standard values of the ellipsoid and geoid

Latitude and Longitude

Longitude line (Meridian)N

S

W E

Range: 180ºW - 0º - 180ºE

Latitude line (Parallel)N

S

W E

Range: 90ºS - 0º - 90ºN(0ºN, 0ºE)

Equator, Prime Meridian

Latitude and Longitude in North America

90 W120 W 60 W

30 N

0 N

60 NAustin: (30°N, 98°W)

Logan:(42°N, 112°W)

Lincoln:(40°N, 96° 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

Curved Earth Distance(from A to B)

Shortest distance is along a “Great Circle”

A “Great Circle” is the intersection of a sphere with a plane going through its center.

1. Spherical coordinates converted to Cartesian coordinates.

2. Vector dot product used to calculate angle from latitude and longitude

3. Great circle distance is R, where R=6370 km2

X

Z

Y•

AB

Longley et al. (2001)

)cos(coscossin(sincos 2121211 R

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

Types of Projections

• Conic (Albers Equal Area, Lambert Conformal Conic) - good for East-West land areas

• Cylindrical (Transverse Mercator) - good for North-South land areas

• Azimuthal (Lambert Azimuthal Equal Area) - good for global views

Projections Preserve Some Earth Properties

• Area - correct earth surface area (Albers Equal Area) important for mass balances

• Shape - local angles are shown correctly (Lambert Conformal Conic)

• Direction - all directions are shown correctly relative to the center (Lambert Azimuthal Equal Area)

• Distance - preserved along particular lines• Some projections preserve two properties

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°

ArcGIS 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

National Hydro Data ProgramsNational Elevation Dataset

(NED)National Hydrography Dataset

(NHD)

Watershed Boundary DatasetNED-Hydrology

Data Sources for GIS in Water Resources

What is it?What does it contain?What is the GIS format?Where would it be obtained

http://www.ncgc.nrcs.usda.gov/products/datasets/statsgo/

1:250,000 Scale Soil Information

http://www.ncgc.nrcs.usda.gov/products/datasets/ssurgo/

1:24,000 scalesoil information

SSURGO:County Level Digital Soil Maps

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

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

http://www.ncdc.noaa.gov/oa/ncdc.html

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

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

Spatial Analysis Using Grids

Vector and Raster Representation of Spatial Fields

Vector Raster

Discrete space view of the world Continuous space view of the world

Numerical representation of a spatial surface (field)

Grid

TIN Contour and flowline

Six approximate representations of a field used in GIS

Regularly spaced sample points Irregularly spaced sample points Rectangular Cells

Irregularly shaped polygons Triangulated Irregular Network (TIN) Polylines/Contours

from Longley, P. A., M. F. Goodchild, D. J. Maguire and D. W. Rind, (2001), Geographic Information Systems and Science, Wiley, 454 p.

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 o

f row

s

Cell size

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

Extent

Spacing

The scale triplet

Support

Extent: domain which is being madeSpacing: distance between measurementsSupport: footprints for what those measurements are…

Spatial Generalization

Central point ruleLargest share rule

Raster Calculator

Cell by cell evaluation of mathematical functions

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

Nearest Neighbor Resampling with Cellsize Maximum of Inputs

40 50 55

4347

414442

42

100

m

4

2 4

6150

m

40-0.5*4 = 38

55-0.5*6 = 5238 52

41 39

42-0.5*2 = 41

41-0.5*4 = 39

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.

Hydro Networks in GIS

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

Streams

Watersheds Waterbody

Hydro Points

Arc Hydro Framework Input Data

!(

!(

!(!(

!( !(!( !(!( !(!(

!(!(

!(!(!(!( !(!( !(!( !(

!( !(!( !( !(!(!(

!(

!(!( !(!(!( !(

!(!(

!(!( !(!(!( !( !(!(!( !(!(

!( !(!(!( !(!(

!(!(!(!(!(

!(

!(!(

!(

!(!(!(

!(

!(

!(

!(!(!(

!(

Feature

Waterbody

HydroIDHydroCodeFTypeNameAreaSqKmJunctionID

HydroPoint

HydroIDHydroCodeFTypeNameJunctionID

WatershedHydroIDHydroCodeDrainIDAreaSqKmJunctionIDNextDownID

ComplexEdgeFeature

EdgeType

Flowline

Shoreline

HydroEdge

HydroIDHydroCodeReachCodeNameLengthKmLengthDownFlowDirFTypeEdgeTypeEnabled

SimpleJunctionFeature

1HydroJunction

HydroIDHydroCodeNextDownIDLengthDownDrainAreaFTypeEnabledAncillaryRole

*

1

*

HydroNetwork

*

HydroJunction

HydroIDHydroCodeNextDownIDLengthDownDrainAreaFTypeEnabledAncillaryRole

HydroJunction

HydroIDHydroCodeNextDownIDLengthDownDrainAreaFTypeEnabledAncillaryRole

Arc Hydro FrameworkData Model

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

45.02304867

50.030

5267

Slope:

Hydrologic Slope - Direction of Steepest Descent

55 a 47 b 48 c

67 d 56 e A

49 f

52 g 45 h 42 i

cingx_mesh_spa * 8i) 2f (c - g) 2d (a

dxdz

100 * 842) 49*2 (48 - 52) 67*2 (55

dxdz

06625.0dxdz

acing y_mesh_sp* 8 c) 2b (a -i) 2h (g

dydz

100 * 8 48) 47*2 (55-42) 45*2 (52

dydz

0163.0dydz

0.06625

-0.0163 0682.00163.006625.0 22

(i) standard slope function

standard slope = 0.0682atan(0.06625/(-0.0163)) = -76.2o + 180 = aspect of 103.8o

Aspect = 103.8 o

Grid cell size 100m

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

166

25

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

55

11

1

3

22

334 4 4

4 45

5

666

Stream Segments in a Cell Network

ArcHydro Page 74

Stream links grid for the San Marcos subbasin

172201

204

202

206203

209 Each link has a unique identifying number

Vectorized Streams Linked Using Grid Code to Cell Equivalents

VectorStreams

GridStreams

ArcHydro Page 75

DrainageLines are drawn through the centers of cells on the stream links. DrainagePoints are located at the

centers of the outlet cells of the catchments

ArcHydro Page 75

Same Cell Value

Catchments for Stream Links

• 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

Catchment, Watershed, Subwatershed.

ArcHydro Page 76

Watershed outlet points may lie within the interior of a catchment, e.g. at a USGS stream-gaging site.

Catchments

Subwatersheds

Watershed