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