Creating a Geographic Footprint from LIDAR Data in ArcGIS
DeAndra S. Hayes, Nina Vu, and Wendy Zhang
Computer Information Systems Graduate Studies Program
Southern University at New Orleans New Orleans, LA USA
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Abstract— LIDAR (Light Detection and Ranging), like radar, is an active remote sensing system. This technology involves the use of pulses of laser light directed toward the ground and measures the time of pulse return. The return time for each pulse, back to the sensor is processed to calculate the variable distance between the sensors and the various surfaces present on the ground. The project is to create a geographic footprint from LIDAR data, using Microsoft Visual Basic for Application (VBA) and ArcGIS. VBA is an across product language that allows users to create complex and fully functional applications. The problem at hand was to find the line beginning and the line end of each scan line and mark those points to extract the polygon outline. The maximum and minimum points, which will declare the left or right outermost points of the sample image, were found. These x and y connecting points cannot have openings or dangling lines when being shaped into a polygon. In order to avoid unwanted shapes, the programs used to arrange points to draw a polygon of the selected data were written. VBA was used to write the program codes and then the codes were integrated with ArcGIS software. This paper demonstrates specific examples regarding the creation of geographic footprint from LIDAR data in ArcGIS. The end product of the project can be useful for anyone working with LIDAR data.
Keywords: LIDAR, ArcGIS, Remote Sensing
I. Introduction
Light Detection and Ranging (LIDAR) is an active remote sensing system. This technology involves the use of pulses of laser light directed toward the ground and measures the time of pulse return [5]. The LIDAR instrument transmits light out to a target, then measures distance, speed, and rotation with respect to target. The transmitted light interacts with and is changed by the target. The laser’s light is scattered by molecules and particles in the atmosphere [6]. Light scattered back towards the LIDAR system is collected by a telescope. It is then measured by a photon detector and digitized using a computer recording system. The signal of the laser is recorded as a function of time which is the travel time for laser light from LIDAR to the target and back [3].
ArcGIS was developed by Environmental Systems Research Institute (ESRI). It is a software program for creating, managing and analyzing geographic data. ArcMap, ArcToolbox, and ArcCatalog are the core products of ArcGIS [2]. ArcMap is used for such purposes as generating, preserving, and evaluating maps. ArcToolbox, a subset of ArcMap, provides data conversion and geoprocessing tasks. ArcCatalog, also a subset of ArcMap, manages spatial data. The technology framework of ArcGIS is known as ArcObjects; it can customize and extend the capabilities of ArcGIS using the ArcObjects facilities. It is based on Microsoft’s Component Object Model (COM) specification. For any given platform (hardware or operating system and combination), COM defines a standard way to lay out virtual function tables in memory, and a standard way to call functions through the virtual tables. Thus, any language that can call functions via pointers (C, C++, Smalltalk, and Visual Basic) can be used to write components that can interoperate with other components written to the same binary standard [7].
A. Statement of the Problem
LIDAR data is stored in an ASCII file format and is used to capture the LIDAR mapping where each row of the file represents a single LIDAR return. Each observation is the coordinate and elevation based on the geometry and timing of a single laser pulse that was sent, reflected and received [8]. Each coordinate is made up of three numbers x (longitude), y (latitude), and z (elevation) [4]. The problem with LIDAR data is its large file size that can range over a million points. These files cannot be read directly into ArcGIS because the points would overwhelm the RAM and the user would not be able to see the overall geographic coverage of the specified area. Microsoft EXCEL is used to open the ASCII LIDAR file and extract only the x and y coordinates and save it as a comma delimited textfile. Unfortunately EXCEL is limited to 65,536 lines so this is the maximum number of LIDAR points it can import from a file [1].
In addition, the method of the data collection capabilities of LIDAR does not provide a geographic footprint of the data file. When the footprint is displayed, it comes up as a flight path and the actual data is located somewhere in the middle of the path. By creating a polygon outline of the actual LIDAR data, appropriate information can be obtained about the LIDAR data. In order to avoid unwanted shapes, a program needs to be written to arrange points to highlight a polygon shape around the selected data. This will allow the GIS analyst to display an accurate footprint of the LIDAR data to determine coverage of an area [7].
B. Objective
The objective of the project is to describe the creation of a geographic footprint from LIDAR data using Microsoft Visual Basic and ArcGIS to determine coverage of an area. Visual Basic for Application (VBA) is an across product language that allows users to create complex and fully functional applications. ArcGIS is used to create layers of detailed information on a map. This footprint declares the left and right outermost points of the sample image. This project is to create the outer boundary of the sample image by allowing the user to submit a request and retrieve the polygon shape. The end result of this project will be useful to anyone working with the ArcGIS and LIDAR data by using VBA to develop a technique for a faster way to determine the boundaries around sample geographical images.
II. Methodology
A. Data Gathering
A LIDAR shapefile containing over 65,536 points was used as the sample LIDAR data set. The sample data is raw LIDAR data and limited to LIDAR flight paths of only north or south direction. This particular file is sequential by starting with a single x-y coordinated pair of a zig zag pattern. The file format is a simple ASCII file with three main elements: longitude (X), latitude (Y), and elevation (Z).
ArcMap is able to read these ASCII files as long as they are comma delimited and have an *.dbf or *.txt extension. If the ASCII file is not comma delimited, it can be converted into this segmentation, for example, through Microsoft EXCEL for less than 65,536 points or Microsoft ACCESS.
ArcCatalog is used to open the comma-delimited textfile and create a table to extract the x and y coordinates.
B. Method of Analysis
The LIDAR data is converted to a shapefile. A shapefile is a nontopological file format for graphical data used by ArcMap. A program was written in Visual Basic to determine the left and right boundaries of the sample image. The comma-delimited textfile is imported into the Visual Basic program fifteen lines at a time. Position eight is considered the middle point. The program checks to see if the values are less than or greater than the middle value. If the value is less than the middle, it is considered a minimum point and is located on the left side of the image. If the value is greater than the middle value, it is considered a maximum value and is located on the right side of the image. The program also displays a ‘1’ if the value is on the left side and a ‘2’ if the value is located on the right side in the output file that contains the x and y coordinates of the image. [See Figure 1.]
Figure 1 ASCII Text File of 65,536 Points
C. Method of Presentation
An extension of ArcMap called Xtools constructed a polygon from the selected points in ArcMap. This code created a simple polygon out of the points to depict the sample image. It was originally developed as a set of useful tools for vector spatial analysis, shape conversion and table management for ArcView GIS by Mike DeLaune. XTools extension was then converted to ESRI ArcGIS 8.x environment and now is redesigned, enhanced, extended and represented as XTools Pro to get to the newer level of functionality and performance.
A shapefile is a nontopological fill format for graphical data. An icon was created in order to have the Visual Basic code depict the boundaries of the sample image automatically. Once the Visual Basic code is compiled, the newly depicted maximum and minimum points are stored in an output file. In the XTools, “Make One Polygon from Points” converts point features of a point layer to one single part polygon and stores it in a new shapefile.
The new shapefile contains only the left and right outer points. [See Figure 2.] The points in the shapefile that contain the boundary points are in exactly the same order as the vertices in the resulting polygon.
Figure 2: Left and Right Boundaries
III. Findings
This research project determined the outer boundaries of a sample image of a LIDAR data by first converting it from a delimited text file. It then determines the image’s boundaries by depicting the maximum and minimum points by calculating statements in Visual Basic and placing a ‘1’ for left boundary and ‘2’ for right boundary in the output file. The Xtools used the coordinates of the 1 and 2 values to determine the direction in which the polygon would be drawn.
Based on the result of this project the information obtained is beneficial to LIDAR data users. The left and right side of the LIDAR footprint was created around the LIDAR image of points by depicting its boundary and locating the outer points. A program was created to determine the outer boundary and display the sides of the footprint automatically. This saves time and results in accurate data for LIDAR data users.
An accurate LIDAR data set was tested that contained over 1 million points. The boundaries were detected through the Visual Basic program and its boundary points were displayed in ArcMap, [See Figure 3] and a footprint was drawn instantly. [See Figure 4.]
Figure 4: Footprint of Million Points
IV. Conclusion
The purpose of this research was to determine the outer boundaries of a sequential image of a LIDAR data set. The LIDAR is saved as delimited textfile. Visual Basic code was used to determine the most accurate outer boundaries of this sequential image. It was transformed into a shapefile in order to be seen and manipulated in ArcMap.
A. Directions for Future Research
LIDAR has become a fixture of present day mapping missions, providing cost effective means to achieve accurate results. Continued improvements in the technology of the equipment and processing tools include better accuracy and feature extraction, which will assist in more effective update of maps.
Even though the objective was reached in this research project, there is still room for improvement. Most LIDAR lasers use near-infrared (NIR) radiation. Certain materials and surfaces, such as water, asphalt, tar, clouds, and fog absorb NIR wavelengths causing null or poor returns. LIDAR does not reflect from water. This creates dead zones where no elevation can be calculated. Therefore, future research has to come up with a solution to solve this problem so that elevation can be depicted and boundaries can be drawn around water.
Acknowledgment
The work is supported by SUNO/FAR (Southern University at New Orleans/NASA Faculty Award Research) program in NASA Stennis Space Center. Special thanks to Ms. DeNiece Guest and Mr. Kenton Ross of Remote Sensing Directorate,
Lockheed Martin Space Operations Stennis Programs for the mentoring and helping.
References
[1] Department of Physics and Atmospheric Science Dalhousie University, “What is LIDAR “. March 2004.
http://aolab.phys.dal.ca/lidar/
[2] Environmental Systems Research Institute (ESRI), Inc., “Getting to Know Desktop GIS”. 1995
[3] Kennedy, M, “The Global Positioning System and GIS: An Introduction”. Chelsea, MI: Ann Arbor Press, Inc, 1996.
[4] Lillesand, Thomas M., Kiefer Ralph W. & Chipman, Johnathon W., “Remote Sensing & Image Interpretation”. 5th edition. New York: John Wiley and Sons, 2004.
[5] Logicon Spectral Imagery Training Center, “Multispecial Imagery Reference, Guide”. Logicon Geodynamics, Inc. 1997.
[6] National Aeronautics and Space Administration, “LIDAR Tutorial Page”, August 1999.
http://www.ghcc.msfc.nasa.gov/sparcle/sparcle_tutorial.html
[7] Razavi, Amir H., “ArcGIS Developer’s Guide for VBA”. Delmar Learning., 2002.
[8] Lo, C.P., Yeung, Albert K.W., “Concepts and Techniques of Geographic Information Systems”. Prentice Hall, 2002.