Why maps are made

• Anthropological consideration of the power relationships that are involved in mapping.
• Natives to a place don't really need maps because they know where things are.
• Maps are for the inexperienced, they are a way of concretizing and forming knowledge of a place.
• Thus mapping became particularly important in periods of human history associated with the rise of the nation state and colonies. People needed to figure out trade routes, illustrate ownership over places, and coordinate activities at a distance. Grassroots mapping unsettles some of these dynamics by allowing locals to easily produce maps of their areas which can be used to counter official maps.

image: "Félix Nadar", Gaspard-Félix Tournachon

Background and discussion of aerial imaging

• A look at the history and timeline of aerial photography.
• Discussion of different methods from the history of aerial photography to those in use today.
• Excellent historical review and slide show: http://www.papainternational.org/history.html

Principles of aerial imaging

Introduction to aspects of aerial imaging such as GPS, map scale, projections, spatial and image resolution, image sensors, field of view, nadir & zenith, stereoscopy, electromagnetic spectrum, spectroscopy.

Oblique and Vertical Imagery

A discussion of the different characteristics of oblique and vertical imagery. Scale is preserved with vertical mapping while oblique imagery provides a different visual perspective for interpreting information from the scene.

Balloons and Kites: an accessible method to a larger user group

Balloons and Kites are introduced as ground based methods for aerial image data acquisition. The methods are both low cost and accessible to ground based mapmaking. The activities are non-hazardous by design with safety protocols, and significantly less hazardous than alternative methods for imagery acquisition. Persistent monitoring capabilities are discussed. Applications are considered in discussion such as; on-demand mapping, high resolution opportunities, crisis mapping, VGI, and international and political implications to public citizen mapmaking.

Unmanned aerial imaging systems are less accessible to the general public due to required training, higher costs, regulatory situations, and and increased hazard and safety concerns.

Each method is useful in different weather conditions.

Balloon mapmaking

The tethered balloon as an aerial imaging platform. The platform is highly maneuverable, and extensible. The system is not complex, and results can be reproduced through instruction and suitable flying conditions.

Kite aerial photography

The kite method of aerial acquisition has different characteristics than using a balloon. There is no cost of helium, but the activity requires more space and is somewhat less maneuverable.

Planning for mapmaking

What do we know about our area of interest? What maps are available and or what do we intend to map? If mapping for another community or group of people, their input is part of this discussion. Discussing someones personal, historic knowledge of an area and how it has or has not changed over time also contributes to effective map planning.

Site and situation planning for the field mapping

The topography of the target mapping area is considered. The intended spatial resolution and associated image footprint are realized. A vector flight plan is constructed that is purposed to provide full coverage of the area of study.

Spatial Resolution and Image Resolution

In digital imaging, image resolution is the number of pixels in a linear inch, pixels per inch (or PPI), also referred to as dots per inch (DPI). The more pixels, or “dots,” per inch, the higher the image resolution. In a digital map, the spatial resolution is a measurement of the actual real world size of a singular pixel. For example, If a map has a 1m spatial resolution, the smallest distinguishable objects are 1m in size. In this case smaller features such as street lines are indistinguishable.

Image size can be described as the number of pixels on the x/y axis of the image.

Field of view and angle of view

The angle of view of a photo is a proportion of the scene that the sensor is pointing at as an angle. Point and shoot cameras are typically below 50 degrees, while wide angle lenses and fish-eye lenses are up to 180 degrees angle of view.

Field of view (also known as field of vision) is the distance covered of the visible extent by a projection at a given distance.

http://wiki.panotools.org/Field_of_View

Calculating the resolution and Swath size of the Imaging platform

Essentially, the image resolution of an aerial image is determined by the size of the horizontal field of view (HFOV) divided by the number of horizontal pixels in the image. The HFOV is a dynamic value that can be determined for any known camera position. Calculating HFOV will be discussed in detail, include a basic DIY method below.

Discussion of focal length and lens varieties such as rectilinear and fish-eye and how they relate to HFOV.

Activity: basic lens calculation technique

Step 1 : Set your camera with intended lens and either no zoom or a static value.

Step 2: Stick a tape measure on a flat wall in a horizontal direction. Use a level if possible.

Step 3: Stand perpendicularly to the wall and aim at the middle of the tape measure. It is ok if some of the tape is not in the frame. It is better than having “white space” on the left or right. Before you pull the trigger make sure you will be able to measure exactly the distance to the wall. The best way is to use a tripod. If you do not have one (go buy one...) you can do the following : make a pendulum with a very flexible string and a weight and hold it right below your camera. Look down and mark on the ground the camera position right after you took the picture. Or position the camera on a table or some type of raised platform.

For Thermal cameras, you will not be able to “take a picture” as described above. Instead of a tape measure, apply 2 fingers on the wall at a known distance from each other and on a horizontal line. You can also take a picture of a door or window, it will have a significant spectral signature for width measurement.

Step 4: Measure the distance between the wall and the camera using the tripod position or the mark on the ground. In this case case the distance is 105.5 cm from the wall.

Step 5: Open the picture on your computer, it should look like this : Image reduced in size, measuring tape visible at 100%

Now we are going to count how many centimeters we have in our picture : Here it starts at 33.5 cm and finishes at 146 cm so we have 146-33.5= 112.5 cm in the frame.

Step 6: now with some very basic math we are going to calculate a distance (height) / width (swath) ratio. In my case 112.5 / 105.5 = 1.06635. As often when using default settings we almost have a 1 to 1 ratio. We are now going to apply this ratio to a regular mission.You plan to fly at 200 meters above ground your swath will be 200 x ratio, in my case for example it will be 200 x 1.06635 = 213,27 meters. You can now apply this ratio and find out what is the best elevation to cover your area of interest.

Step 7: Inspect the results of image overlays to improve the ratio after the initial shooting by measuring the actual swath size with associated GPS values.

For example the picture I am going to use for calibration was taken at 11:05:50. Aircraft absolute (GPS) height was 1897 meters. Ground altitude below this point is 610 meters. Therefore height above ground (was distance to the wall above) is 1897 - 610 = 1287m. Next, overlay of the corresponding image in Google Earth.

Measuring the width of the overlayed image = 571m. that was previously how many cm of tape measure we had in the frame) Therefore my ratio is 571 / 1287 : 0.4437 In this case it is below 1. This means that I used a zoom.

Step 8: Calculating theoretical image resolution. From the EXIF data the image is 3648 x 2736 pixels. To find image resolution, 3648pixels represent 571m horizontal ground distance. Therefore 1 pixel measures 571 / 3648 = 0.1565 meters.Image resolution is about 15 cm.

This same ratio can be used to determine the height of the camera without GPS by from the HFOV ground distance of an image.

Advanced planning for higher precision

Discussion of additional measures for achieving higher precision in calculating the image footprint. In depth discussion of HFOV at epaper press Basic online lens calculator Lens Equivalence calculation and more tools

Image footprint characteristics for Canon 5D MKII camera imaging at 500’ AGL.

Comparative Matrix of several popular consumer cameras

Spatial resolution and image definition

You can have a 30 cm image look “better” or more detailed than a 10 cm one. This is the difference between “spatial resolution” and “image definition”. Spatial resolution says how many cm a pixel represents but image definition is more about quality, it takes into account potential blur (from vibrations...), contrast and luminosity (you want your histogram to cover the whole spectrum, use as many bits as possible)....

Image definition gives the quality of the image, it is what makes you say, “wow that is a good one” before you even find out the spatial resolution.