An Open-Source, Object-Oriented General Cartographic Transformation Program (GCTP)

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An Open-Source, Object-Oriented General
Cartographic Transformation Program (GCTP)

• Michael S. Williams, Michael P. Finn, and Robert
  A. Buehler
• United States Geological Survey
• National Geospatial Technical Operations Center

Presenting author Now with University of
Missouri Rolla, Department of Computer Science
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Outline

• Motivation
• Goals
• Background
• Design
• Class Structure
• Interface
• Adding New Projections
• Conclusions
• References

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Motivation

• Object-Oriented software development popularity
  growing
• Many existing libraries written in procedural
  languages (C, for example)
• Many current solutions involve using these
  procedural libraries within object-oriented
  frameworks (i.e. wrapping them)
• Preferable to have a natively object-oriented
  library to use

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Goals (Dos and Donts)

• Dont completely reinvent the wheel
• Dont just make a wrapper
• Do use existing algorithms
• Do make it simple to use
• Do make it easy to extend

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Background

• General Coordinate Transformation Package (GCTP)
• Originally written in Fortran
• Later converted to C
• Still very widely used

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Background

• Original GCTP interface very cumbersome.

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Background

• Original interface uses 19 parameters!
• Calling the gctp function results in much
  redundant code execution.
• How can we improve this?

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Design
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Design

• Simple Design
• Each supported projection contained in a single
  class.
• Each object inherits from the base class
  Projection.
• Projection class provides a common interface.

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Design

• Whats old and whats new
• Old
• Still supports the use of the 15 element
  projection parameter array.
• Core projection algorithms exactly the same.
• New
• Supports the use of get / set functions to view
  and modify projection parameters.
• No more 19 parameter function calls!!

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Design
Parameter Array
Variable Declarations
Output Units
Datum
Object Instantiation
Forward Transformation
Inverse Transformation
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Design
Setting Parameters
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Adding New Projections

• Basic Steps
• 1.) Derive a new class from Projection
• 2.) Implement the protected _init(), _forward(),
  and _inverse() functions.
• 3.) If the new projection uses specific entries
  in the parameter array, implement the
  _loadFromParams() function.

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Adding New Projections (example)
Constructors
Core Projection Function Declarations
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Adding New Projections (example)
Transformation results must be stored in
m_x_coord and m_y_coord, which are inherited from
the Projection base class.
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Adding New Projections (example)
First, a call must be made to the base class
parameter loading function
Here, the UTM specific parameters are pulled from
the parameter array.
Here, the UTM projection class member m_zone is
assigned a value based on the lat/lon values
pulled from the parameter array.
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Conclusions

• We have developed an object-oriented version of
  an existing, widely used projection library.
• We have made this library much easier to use than
  the previous version. We have also made it easy
  to extend.
• Currently, the library is still in beta testing.
  Further testing needs to be done to check
  projection accuracy.
• For further information (download available soon
  for the latest version), please visit the
  following website
• http//carto-research.er.usgs.gov/

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An Open-Source, Object-Oriented GCTP
http//carto-research.er.usgs.gov/

• Michael S. Williams, Michael P. Finn, and Robert
  A. Buehler
• United States Geological Survey
• National Geospatial Technical Operations Center