How accurate is the imagery your vendor just delivered? Do you know? Spatial Informations Solutions (SIS) recently released Accuracy Analyst to answer that question. Directions Magazine
interviewed SIS President Chuck O'Hara to learn about the product and
its place in organizations on the receiving end of imagery data.
Directions Magazine (DM): What is Accuracy Analyst? Is it standalone software?
Chuck O'Hara (CO): Accuracy Analyst is a software solution for
determining the locational accuracy of georeferenced image data that
producers, government agencies and users around the world employ on a
daily basis to create, update and refine vital map data and products.
Accuracy Analyst runs on a typical PC desktop (Windows XP or higher)
and provides a streamlined and high performance interface as well as an
easily mastered workflow that enables users to evaluate enormous image
data collections without requiring enormous resources or investment.
DM: SIS notes that "Accuracy Analyst is the first software solution
to determine location errors in the image data used to produce maps."
Can you further explain that? In particular, what kind of errors are
CO: Accuracy Analyst provides standards-compliant quantification
and direct understanding of the locational error in georeferenced image
data. This error is the difference between locations of well-defined
objects in the image and their actual location on the ground. Accuracy
Analyst is the first software solution to quantify the locational
accuracy of georeferenced image data. This software is easy to use and
provides statistical results that are compliant with NSSDA [National
Standard for Spatial Data Accuracy] standards to test accuracy to a 95%
What this means is that Accuracy Analyst allows the user to input
surveyed locations (at least 20 points - well-distributed points are
recommended throughout the project area) that are visible on the base
map images, automates the process of extracting and comparing
photo-derived positions, and results are statistically analyzed to
provide standards-compliant accuracy statistics. Accuracy Analyst also
provides highly useful plots of circular error distribution (useful to
show error patterns or bias) and a vector offset plot that shows the
geographically varying nature of errors by vector arrows of direction
and relative magnitude of errors across the study area. The combination
of statistical results, tabular output and graphical plots deliver
quantification and understanding. Accuracy Analyst also provides
snapshots of each survey location and highlights the position of the
survey location and the photo-identified location of each feature
surveyed along with the coordinates for both locations and the amount
of error at that location. All this information is concisely compiled
into an output report that is date- and time-stamped and may be
regenerated at any time. ï¿1⁄2
DM: What information about the imagery is needed to use the software? In what form should the imagery be?
CO: In order to perform an assessment of locational accuracy, it is
vital to gather ground truth locations by a method that is at least
three times more accurate than the stated accuracy of the image data
being tested. Highly accurate survey checkpoint locations are an
important part of any accuracy assessment and all checkpoints should be
documented and field photos should be taken at all locations and
organized to facilitate analysis of checkpoints and identification of
features in the image data being evaluated.
To begin the analysis of image data accuracy, the user needs to know
the format of the image data, the projected coordinate system and datum
for the data, and the location of the data storage. The user also needs
to have access to information about the data producer/vendor, the
ground resolution of the image data, and should have information about
the sensor used to collect the data. All of this information is entered
into a few simple lines that provide documentation about the Accuracy
Analyst project and are included in the final report printout.
Accuracy Analyst supports a wide variety of data types including the
most common industry standard image formats. The most common image
format is geoTiff. Accuracy Analyst also works on large mosaics that
are usually available as compressed MrSID or ERMapper EZW formats.
Accuracy Analyst currently supports multi-band and panchromatic image
data with best results in 8-bit format data. Enhanced capabilities for
designating colors (red, green and blue) for image band handling is
being built into a "New Features Release" that will be available around
the end of September.
The results of analyzing a 3.8 GB MrSID image county-wide mosaic of
Butler County, Ohio. The file contains over 1,000 individual images
collected at 6-inch resolution. Many counties use mosaics such as this
for mapping. This report illustrates how a user could compare a variety
of data sources for an area and determine from available sources which
actually are the"best available data." (Click for larger image)
DM: Is accuracy information for imagery typically delivered with the
data from the provider? Is independent confirmation typically done by
CO: Accuracy information is not typically delivered from providers
other than in a letter or product documentation that describes the
process used to create data and the typical accuracy that the
production process delivers. Customers can place requirements for
accuracy testing as well as "Independent Validation and Verification"
of products, but in the past this has created significant costs and
delays. With Accuracy Analyst, a set of survey checkpoints can be
efficiently acquired and used within Accuracy Analyst to produce a full
accuracy assessment quickly, concisely and in a unique manner that
fully documents data accuracy and delivers easily comprehended results,
meaning and understanding to the producer and the customer. Accuracy
Analyst provides a standards-based common ground for delivering data
products that enable confidence in delivery and acceptance of base map
DM: How is accuracy information currently compiled?
CO: Accuracy information is currently compiled in an "all manual"
process. Accuracy Analyst fully automates the process. Highly accurate
survey checkpoint locations are an important part of any accuracy
assessment. The current process considers the survey locations,
compiles the X and Y coordinates into a spreadsheet, loads image after
image into a conventional GIS, finds corresponding photo identifiable
points, captures screen shots of point locations, takes the locations
off of the GIS interface manually, puts the image-derived location on
the spread sheet, and calculates the offsets for each survey
checkpoint. When this has been completed for all the checkpoints and
all offsets are calculated, accuracy statistics are calculated using
the spreadsheet and results are compiled into an accuracy report. Using
current methods, if the analyst wants to include photos, screen grabs
of points, or information, it must all be done manually. Furthermore,
using current methods, the data compilation, analysis and report
generation process can take up to four or five days for a large
Accuracy Analyst does all of this automatically. It takes what has
traditionally been a multi-day process and finishes it in as little as
five to 10 minutes. Accuracy Analyst pays for itself in the first use
and produces results that are far superior to any manual process. The
return on investment is enormous considering the cost and time savings
on every use of Accuracy Analyst.