Digital Aerial Imagery

Calibration Range Requirements

EROS

Remote Sensing Technology Project

September 2008

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[1.0 Scope 3](#scope)

[2.0 Overview 3](#overview)

[3.0 Range Implementation Requirements
4](#range-implementation-requirements)

[3.1 Range Specifications 4](#range-specifications)

[3.2 Ground Point Specifications 5](#ground-point-specifications)

[3.3 Spatial Target Specifications (Optional)
6](#spatial-target-specifications-optional)

[4.0 Range Maintenance Requirements 7](#range-maintenance-requirements)

[5.0 Range Usage Requirements 7](#range-usage-requirements)

[Appendix A: Figures and Tables 9](#appendix-a-figures-and-tables)

[Appendix B: TBR/TBDs 13](#__RefHeading___Toc210549867)

[Appendix C: Acronyms 14](#appendix-c-acronyms)

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# 1.0 Scope 

This document defines the United States Geological Survey (USGS)
requirements for building, maintaining, and using standardized In Situ
Calibration Ranges (ISCRs) for the purpose of calibrating and/or
validating aerial sensor systems. Calibration over these ranges is
intended only to be geometric calibration, the determination of the
systems interior orientation parameters including lens distortion or
some subset thereof. The calibration can then range from a full,
from-scratch calibration to the tweaking of a single, previously
determined parameter. The ISCRs could also be used for such things as
validation of geometric calibration and system response over time of
aerial digital sensor systems and, in some limited circumstances, be
used to support a boresight calibration.

The development of these ranges is directed primarily towards aerial
camera system owners and manufacturers of flight-worthy digital cameras.
Secondarily, the ISCRs are directed towards any other safe (i.e.,
non-injurious to plants and/or animals) sensor system and towards
contracting officials who may require the calibration results from the
primary users for assessment purposes.

Unless prior approval is given by the USGS, the results of an in situ
calibration over one of these ranges would not be considered as a
replacement of any prior calibration of an aerial camera system.

# 2.0 Overview

The ISCRs will be distributed throughout the United States to provide
easy access for everyone. They will be uniform in ground coverage,
rectangular (TBR-0) in shape, and oriented in a cardinal direction
(i.e., either from north to south or from east to west). The rectangular
shape will provide flexibility in the planning of flight lines for the
disparate image footprints of the various aerial sensors to ensure an
adequate number of images for a proper calibration.

The ranges will cover non-flat terrain, have precisely surveyed ground
points, and have a Continually Operating Reference Station (CORS)
readily accessible. The ground points will be either signalized control
points for computing a calibration bundle solution or image
identifiable/signalized check points to evaluate the "goodness" of the
bundle solution. This will provide sufficient data for users to
determine or validate, and the USGS to assess, the camera's calibration.

In addition to these physical attributes, the ISCRs will have
accessibility attributes such as open airspace, low air turbulence, no
dangerous vertical obstructions, and convenient ground access to the
range for routine maintenance. It is of primary importance that the
users of the range have a convenient and safe environment to collect the
data required for the determination or validation of the sensor's
calibration.

# 3.0 Range Implementation Requirements

There will be two co-resident ranges within each physical range;
usually, one for larger format digital (and film mapping) cameras and
one for smaller format cameras. Depending on the purpose of the
over-flight and for flexibility, any camera system can be flown in
either cardinal axis direction and in either of the two range layouts to
produce four distinct range options. See Appendix A for Tables and
Figures providing support for the requirements in this section.

## 3.1 Range Specifications

1)  The extents of the ranges were derived from the characteristics of
    typical camera systems (Figure 3.1-1: Range Extents and Placement).

    a.  The larger range shall have a ground extent of 1750 (TBR-1) by
        2000 (TBR-1) meters.

    b.  The smaller range shall have a ground extent of 600 (TBR-1) by
        750 (TBR-1) meters and be totally within the larger range.

2)  The orientation of the longer dimension for all ranges shall be
    east/west or north/south.

3)  The terrain variation within any range shall be at least 100 (TBR-2)
    meters (Figure 3.1-2: Terrain Variation Examples).

    a.  Terrain shall not be uniformly sloped over the range's extent.

    b.  The top of a stable manmade object may used in the determination
        of terrain variation if a control point is located on top of
        this object.

    c.  The control point variation shall be at least 75 (TBR-2) meters.

4)  The range area shall be within 40 km of a Continually Operating
    Reference Station (CORS) that operates at a minimum rate of five
    (TBR-3) seconds (one second CORS is preferred).

5)  Ranges should be located in areas that are predominately cloud-free.

6)  Ranges shall be easy and safe to access.

    a.  Ranges shall not be located within 10 (TBR-4) kilometers of
        controlled or restricted airspace (including maneuvers to line
        up for flight lines).

    b.  Ranges shall be located at least 10 (TBR-5) kilometers from
        obstacles over 100 meters in height.

    c.  Ranges shall be located at least 15 (TBR-6) kilometers from any
        source of significant turbulence that is present and active
        during typical over-flight times.

## 3.2 Ground Point Specifications

1)  Each range shall contain at least 50 (TBR-7) signalized control
    points.

    a.  Control points shall be surveyed to a minimum accuracy of 2
        (TBR-8) centimeters at one sigma (i.e., standard deviation)
        horizontally and vertically.

    b.  The range with the larger extent may share a subset of the
        points from the range with smaller extent.

    c.  The points shall be uniformly, but randomly, distributed
        throughout the range such that no less than 20 percent of the
        points (rounded value) fall within any quadrant of the range
        (Figure 3.2-1: Point Distribution Constraints).

    d.  No two control points shall fall closer than the square root of
        the following computation; range length times the range width
        divided by two and one half times the number of points (Figure
        3.2-1: Point Distribution Constraints).

    e.  Control points shall be intervisible to all image exposure
        stations falling within an inverted 45 (TBR-9) degree right
        circular cone with its apex at the control point (Figure 3.2-2:
        Control Point Intervisibility).

    f.  Signalized control points (i.e., panels) shall have an
        acceptable shape and dimensions (Table 3.2-1: Acceptable
        Signalized Control).

        i.  The surveyed location shall be at the precise center of the
            panel.

        ii. Painted panels on asphalt or concrete shall be constructed
            and centered on a surveyed nail, pipe, rod, spike,
            punchmark, drillhole, or other semi-permanent mark.

        iii. Panels constructed from other materials (such as sheets of
             material anchored to a roof) shall be centered over a
             similarly recoverable, semi-permanent surveyed point.

        iv. The white surfaces of the panels shall be painted with that
            specified by standard Diamond Vogel Paints Traffic Paint
            White Oilbase (TB-1501) or equivalent.

        v.  The black surfaces of the panels shall be painted with that
            specified by standard Diamond Vogel Paints Traffic Paint
            Black Oilbase (TB-9502) or equivalent.

        vi. The placement and painting of panels shall conform to local
            laws/restrictions.

    g.  Control point panels shall be located so as to allow for
        convenient maintenance.

2)  Each range shall contain at least 25 (TBR-10) check points to be
    used to evaluate the calibration solution.

    a.  Check points shall have the same specifications as control
        points for accuracy, sharing, distribution, and intervisibility
        (1a-1e and 1g above).

    b.  Check points are not required to be signalized (i.e.,
        permanently marked), but if signalized, they shall have the same
        specifications as control points.

    c.  Once the check points have shown an independent verification of
        the goodness of a range over-flight, they may be used as extra
        control points for computational purposes.

## 3.3 Spatial Target Specifications (Optional)

The determination of relative edge response and image resolution will be
somewhat limited due to the rather small size of the modified Siemens
Star, henceforth known in this paragraph as the star, (Figure 3.3-1:
Modified Siemens Star and Figure 3.3-2: Alternative Modified Siemens
Star, henceforth known in this paragraph as the figures) being
specified. The purpose of the modified star is to provide black to white
edges at a controlled set of orientations such that a relative edge
response can be obtained even when the images are not truly oriented to
a cardinal direction.

1)  The star shall be broken up into four quadrants.

2)  The star shall be oriented to one of the four cardinal directions
    (i.e., north, east, south, or west).

3)  The star shall have the same intervisibility constraints as for
    control points.

4)  The spokes on the star shall be 3.2 (TBR-11) meters in length.

5)  The distance between black areas in adjacent quadrants, except for
    the middle spokes in quadrants 1, 2, and 3 and all spokes in
    quadrant 4, shall have at least one (TBR-11) meter separation to
    provide sufficient room for the determination of relative edge
    response.

6)  Three of the star's quadrants (i.e., quadrants 1, 2, and 3 in the
    figures) shall have spokes with 18 (TBR-11) degree angles.

7)  The spokes of that portion of the star in the fourth quadrant shall
    have angles of 10 (TBR-11) degrees.

8)  The spokes in each quadrant shall have specific orientations to
    provide flexibility in the determination of relative edge response.

    a.  Quadrants one and four shall have no rotation to the cardinal
        direction.

    b.  Quadrant two shall be rotated four degrees clockwise to the
        cardinal direction.

    c.  Quadrant three shall be rotated eight degrees counterclockwise
        to the cardinal direction.

9)  The ends of the spokes shall have specific orientations (Table
    3.3-1: Siemens Spoke End Orientations) to provide flexibility in the
    determination of relative edge response.

10) The white surfaces of the star shall be painted with TBD-1.

11) The black surfaces of the star shall be painted with TBD-1.

# 4.0 Range Maintenance Requirements

The USGS will develop cooperative agreements with groups from
governmental bodies, industry, and academia (i.e., range sponsors) for
joint planning and building of ISCRs. Once built, the sponsors will
perform all maintenance on the range and USGS will assist the sponsors
with any rework of the range.

1)  All routine range maintenance and inspection shall be performed by
    the sponsor of the range.

2)  The sponsor shall maintain a historical record of all inspections of
    the range and any maintenance performed on the range.

3)  The sponsor and USGS shall determine the frequency, scope, and
    reporting of routine inspections.

4)  The USGS shall provide a special maintenance request to the sponsor
    when users of the range report a control point recovery rate of 90
    percent or less.

5)  The USGS shall provide a maintenance advisory to the sponsor when
    the control point recovery rate goes below 95 percent or spatial
    calibration targets become difficult to use.

# 5.0 Range Usage Requirements

The primary use of the ISCRs is for geometric camera calibration which
can be split into at least three levels: full calibration, parameter
tweaking calibration, and calibration validation. Each of these levels
requires varying range resources and varying collections of imagery. In
some situations the range could be used for boresighting a camera system
when the flying conditions (e.g., camera type and flying height) are
appropriate. It will be up to the range user to determine how to best
use the range for the user's purposes.

1)  The range user shall notify the USGS and the range sponsor of a
    planned over-flight.

2)  The range user shall provide the USGS with a copy of the imagery
    used for calibration purposes and the results of the calibration.

3)  At least 80% of the control and check points shall be imaged on two
    or more exposures for calibration purposes.

4)  All control points that are imaged must be part of the calibration
    solution as control points.

5)  Calibration results shall be in ASCII form and shall consist of:

    a.  Derived calibration parameters with statistical measures and
        parameter correlation information.

    b.  Mensuration data.

    c.  Control point data.

    d.  Exterior orientation data.

    e.  Camera data.

6)  The range sponsor shall inspect that portion of the range based upon
    and prior to the planned over-flight if within 5 (TBR-12) days of a
    routine inspection.

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# Appendix A: Figures and Tables

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[]{#__RefHeading___Toc210549867 .anchor}Appendix B: TBR/TBDs

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# Appendix C: Acronyms

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