Lab 7: Photogammetry

Background

This labs objects were to explore the basics of photogametry.  The specific part being looked at included calculating ways to figure out the scale of digital images, measuring perimeters and areas, an introduction to stereoscopy, and orthorectification.

Methods

Scales, Measurements, and Relief Displacement 

Vertical Aerial Photographs

To Calculate the scale of an aerial photograph we use the S= Pd/Gd equation.
Where,
S = Scale
Pd = Photo Distance
Gd= Ground Distance

The following is an example from calculating the scale on an image of Eau Claire, Wisconsin.

S = Pd/GD
S= 2.7'' / 8822.47'

In this case the 2.7 inches was collected by using a ruler measuring the photo distance on the computer monitor.  The next step is to convert to the same units to make the math easier, Therefore the equation looks like S=2.7/105869.64.  To get the Scale divide the numerator and denominator by 2.7'' which will give the result of 1/39211.

In the next example there is an aircraft which aquired the photograph at an alititutde of 20,000 ft above sea lvel with a camera that included a focal length lens of 152mm, and the goal of this example is to figure out the scale of the photograph using the information obtained.

The equation used for this example is S = F/(H-h)
S = Scale
f = focal length
H = altitude above sea level
h = elevation  of the terrain

so,

f = 152mm
h = 769 ft
H = 20,000 ft
Altitude above ground level H': 20,000 ft - 769ft = 19204 ft

Therefore,
S = 0.152m/19204ft = 0.499'/19204'

Divide both the numerator and denominator by 0.499'

S = 1/38485

Measurement of areas of features on Aerial Photographs: 

The next sections shows a lagoon labeled with an x which needs the area calculated.

• Select measure tool from the home tab on the ERDAS Imagine interface
• click "point" from dropdown arrow and select the polygon tool to measure the area of the lagoon
• trace the lagoon creating a polygon and double click when finished
• in the measurement window change the defaults to Hectares and the other needed units

Area: 39.5517 Hectares and 97.7345 acres. 

Figure 1: Map of Eau Claire showing the Perimeter/Area of the Lagoon. 

To obtain the perimeter:

• Use the Measure the perimeter of the feature select polyline tool
• Repeat the outline of the lagoon and doubleclick to finish.  
• Information will be in the window

Calculating Relief displacement from Object Height

Relief displacement happens when an object is not represented in the correct planimetric location because of its distance from the principle point and the height of the overall object itself. 

The taller the object the more displacement it will have, and the farther away the object is from the principal point the more displacement it will have.  

The equation for dealing with relief displacement is

d = (h*r)/H 

d = relief displacement

h = height of the object (real world)

r = radial distance from the top of the object to the center of the principal point

H = Height of the camera above the local datum

In the example given,  the photo is looking at a smoke stack identified by the letter 'A' on the photograph.

H = 3980'

h = (0.5) ( 3209) = 1204.5'' (in the real world): 0.5 was the photo distance

r = 10.5'' (measurement taken with a ruler)  

d = (1204.5'' * 10.5'') / 47760''

d= (1204.5'' * 10.5) / 47760'' = +0.265 above principle point of elevation) 

The tower should be plotted inwards since the displacement is positive. 

Steroscopy

Steroscopy is a method used to enhance an image to display a 3-D view to show variation in elevation.  This part of the lab looks at the elevation of Eau Claire, Wisconsin which was analyzed from the creation of an anaglyph image using a DEM and DSM

Creating an Anaglyph image with the use of a DEM

The first step is to bring in two photographs into seperate viewers.  The first had a one-meter spatial resolution and the other was a DEM of Eau Claire with 10m spatial resolution

The steps are: 

• Click the terrain tab in ERDAS and select Anaglyph to open the anaglyth generation Window
• Input the DEM into the DEM place and the other is the input image.
• Name output file to the correct location
• Set the vertical exaggeration to 1 and leave the rest to default. 

Once the process is complete bring the image into a new viewer and use the polaroid lenses to look at the 3-D image created.  

Creating an anaglyph image with the use of LiDAR derived surface model

Bring in the two images into separate viewers.  The first is a one-meter spatial resolution and the other is a 2-meter spatial resolution. Next, Use the same workflow as above to create the new image which can be looked at using the Polaroid lenses.  

Orthorecectification 

Creating a new project:

Open LPS Project Manager by clicking the Imagine Photogrammetry within the Toolbox tab
Create a new block file
Within the model setup window the polynomial-based pushbroom category and SPOT-Pushbroom were selected.

Horizontal Reference Source

To set the horizontal Reference Source click the "Set" button for the horizontal Reference Coordinate System.

The following were used for this lab:

Projection: UTM
Spheroid name: Clarke 1866
Datum: NAD27 (CONUS)
UTM Zone 11 North
Meters for Units

Adding Images

In the Photorammetry project manager window click images which is on the let side, and add frame.
add the spot image to edit the pushbroom settings then click ok to accept the defaults.

Collecting GCPs

To start collecting GCPs select the Classic Point Measurement Tool option Then the point measurement window will appear.

Now, reset the Horizontal Reference to ensure the image layer and add the orthorecifiedimage, and check the box to use viewer as Reference.  

Figure 2: Showing the GCPs on the image

Figure 3: Table Showing Data entered to collect points. 

Now to add the points to both images click the add GCP button and add it to the Pan Image then click add and continue until desired points are reached.

Then add a second image to the block file, collect the GCPs for that image and perform an automatic tie point collection.  Then Tiangulate the images, and othorcif the images.  Then save the block file and it should create a very accurate looking image as they are lined up.

Figure 4: Images matched together.  

Conclusion:

Creating Steroscopic images to use in orthorecectification can be a very heavy and detailed process to create an image that matches up to nearly 100%.  This is a great tool to use when dealing with multiple images to get them attached together.

Sources

Digital Elevation Model (DEM) for Eau Claire, WI

United States Department of Agriculture Natural Resources Conservation Service, 2010.

Digital elevation model (DEM) for Palm Spring, CA

 Erdas Imagine, 2009.

Lidar-derived surface model (DSM) for sections of Eau Claire and Chippewa

 Eau Claire County and Chippewa County governments respectively.

National Agriculture Imagery Program (NAIP)

 United States Department of Agriculture, 2005.

National Aerial Photography Program (NAPP) 2 meter images \

Erdas Imagine, 2009.

Spot satellite images

Erdas Imagine, 2009.