understanding and using geometry, projections, and spatial ...€¦ · understanding and using...
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Understanding and Using Geometry, Projections and Spatial Reference
Systems in ArcGIS
Rob Juergens, Melita Kennedy, Annette Locke
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Introduction
We present fundamental concepts necessary for the correct and efficient use of geometry and spatial reference APIs
• Geometry types • Spatial references and their properties • How spatial references and geometries interact
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Geometry
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What is a geometry? • Defines the shape of a feature
• Vector representation for top level types In other words, vertices have x, y coordinates • Optional z- (height) and m- (measure)
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Working with and analyzing geometries
• Simple geometry verification - Adhere to a set of rules
• Topological operations - For example, Buffer, Symmetric Difference, Union, Intersection
• Relational operations - For example, Disjoint, Touches, Overlaps, Crosses, Within
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Points Building blocks for all geometries
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Multipoints Each multipoint feature is a collection of points
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Polylines
Elliptical Arc
Points are used to build segments
Composed of paths
Composed of segments
Segments can be four types
Polyline
Paths
Line Bézier Curve
Circular Arc
Points
Segments
A collection of paths
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Polylines
Single part Multipart
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Polygons
Elliptical Arc
Points are used to build segments
Composed of rings
Composed of closed segments
Segments can be four types
Polygon
Rings
Line Bézier Curve
Circular Arc
Points
Segments
A collection of rings
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Polygons
Inner ring
Outer ring
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Non-simple Polygons
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Simple Polygons
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So what? Why do we care if geometries are simple?
• Cannot rely on results from operations using non-simple geometries
• Get an error
• Get incorrect results
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Demo
Why do we care if geometries are simple?
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Spatial References
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Spatial references Key properties
• Coordinate system
- Geographic - Projected
• XY Resolution
• XY Tolerance
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Coordinate systems Geographic (GCS) Projected (PCS)
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What is a coordinate system?
• An agreed upon way to describe locations
• Represents locations - Geographic features - Imagery - Observations such as GPS locations
• Common geographic framework Used to integrate geographic locations from different datasets
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Geographic Coordinate System (GCS)
• Global – 3D spherical surface • Point referenced by longitude and latitude values
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Projected Coordinate System (PCS)
• Flat – 2D surface based on a GCS
X
Y
(0,0)
X > 0 Y > 0
X < 0 Y > 0
X < 0 Y < 0
X > 0 Y < 0
• Point referenced by x, y coordinates on a grid
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Datum
Spheroid
Prime Meridian
Projection Parameters
Projection
Projected Coordinate System
Geographic Coordinate System
Coordinate System
Angular Unit
Linear Unit
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Geographic Coordinate System Well-Known Text (WKT)
GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", 6378137.0, 298.257223563] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", 0.0174532925199433] ]
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What is a datum?
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Projected Coordinate System Well-Known Text (WKT)
PROJCS[ “World_Mercator", GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", 6378137.0, 298.257223563] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", 0.0174532925199433] ], PROJECTION[ "Mercator " ], PARAMETER[ "Central_Meridian", 0.0], PARAMETER[ "Standard_Parallel_1", 0.0], PARAMETER[ "False_Easting", 0.0], PARAMETER[ "False_Northing", 0.0], UNIT[ “Meter", 1.0] ]
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Well-Known ID (WKID) • Every predefined coordinate system has a WKID
- For example, GCS_WGS_1984, WKID = 4326
• WKID < 32767 is EPSG assigned - EPSG Geodetic Parameter Dataset, http://www.epsg-registry.org/
• WKID > 32767 is Esri assigned
- Esri WKID may change - Esri ® EPSG - Old WKID will still work - Example, Web Mercator 102100 ® 3857
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All projections have some distortion
Web Mercator Projection
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What is happening when we project data?
PCS A2
GCS A
PCS A1
(lon, lat)
(x, y)
(λ, φ)
Case 1: Both PCSs contain the same GCS
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Geographic Transformation GCS A
PCS A1
GCS B
PCS B1
Case 2: Each PCS contains a different GCS
What is happening when we project data?
(lon, lat)
(x, y)
(λ, φ)
Projection Projection
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Geographic Transformations (GT)
• Convert from one GCS to another GCS
• Suitable for a particular area • Defined in a particular direction
- For example, NAD27 to WGS84 - All are reversible
• May be more than one applicable GT
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There are 38 transformations between GCS_North_American_1927 and GCS_WGS_1984
Which is best? Depends on the region covered by your data
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Why do we need to transform our data?
European Datum 1950 vs. World Geodetic System 1984
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Demo
Projection between different GCSs
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Resolution and Tolerance
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XY Resolution • Numeric precision used to store x, y coordinate values • All coordinates lie on coordinate grid • Default value is 0.0001 meters or equivalent
- x1 = 5.1234, x2 = 5.1235 stored as unique coordinate values - x1 = 5.12344, x2 = 5.12345 both stored as 5.1234 - Each square in grid is 0.0001 x 0.0001
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XY Tolerance Minimum distance between coordinates when processing features
- Simple geometry validation - Topological operations such as Buffer - Relational operations - Editing operations
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XY Tolerance
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Demo
Merge polygons
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Resolution vs. Tolerance
• Resolution: refers to number of decimal places used to store x, y coordinate values
• Tolerance: minimum separation between features used by some operations - Should never be less than 10 times resolution
• Default resolution = 0.0001 meter or equivalent • Default tolerance = 0.001 meter or equivalent • Highly recommended to use default values!
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What do spatial references have to do with geometries?
• Geometry is a collection of points
• Spatial reference determines - where the coordinates are placed - how the coordinates interact with each other
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How does a spatial reference affect a geometry?
• We need to know where to put the geometry on the map
• A geometry that is simple in one spatial reference may not be so in another spatial reference
Remember, garbage in, garbage out
• An operation on features may give different results depending on the spatial reference
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Demo
Where on the map do I put the geometry?
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How does a spatial reference affect a geometry?
• We need to know where to put the geometry on the map
• A geometry that is simple in one spatial reference may not be so in another spatial reference
Remember, garbage in, garbage out
• An operation on features may give different results depending on the spatial reference
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Demo
What is simple here may not be simple there
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How does a spatial reference affect a geometry?
• We need to know where to put the geometry on the map
• A geometry that is simple in one spatial reference may not be so in another spatial reference
Remember, garbage in, garbage out
• An operation on features may give different results depending on the spatial reference
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An operation on features may give different results depending on the spatial reference
For example, Buffer operation
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Demo
Buffer and Spatial Reference
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Resources • http://resources.arcgis.com/en/help
- Desktop → Guide Books → Map projections - Developer Help
- List of ArcGIS APIs
• Lining Up Data in ArcGIS, Margaret Maher
• ESRI Technical paper: Understanding Coordinate Management in the Geodatabase
• ESRI Technical paper: Understanding Geometric Processing in ArcGIS
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Demos
• ArcMap Demos - DevSummit2014.gdb
- Projection between different GCSs (Project.mxd) - Merge polygons (MergePolygons.mxd) - What is simple here may not be simple there (OneSimplePolygon.mxd)
- QM.gdb - Where on the map do I put the geometry? (QM.mxd)
• JavaScript Demos - Why do we care if geometries are simple? (SimplifyPolygon.html) - Buffer and Spatial Reference (GeodesicBufferWebMercator.html and
GeodesicBufferOther.html)
• Python Toolbox to convert JSON to GDB (MyPythonToolbox.pyt)
All the demos are on GitHub at https://github.com/alocke/DevSummit2014
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That’s all folks!
Don’t forget to fill out the survey
Understanding and Using Geometry, Projections and Spatial Reference Systems in
ArcGIS
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