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Remote Sensing

Remote sensing is merely the science of acquiring information about a surface without physically being in contact with it. It involves the use of technical instruments or sensors to record reflected or emitted energy and then processing, analyzing, and applying that information to determine the spectral and spatial relations of distance objects and materials.

Remote SensingThis is possible due to the fact that the examined objects (such as vegetation, buildings, water, air masses etc.) reflect or emit radiation in different wavelengths and intensities according to their current condition. Modern remote sensing typically involves digital processes but can also be done with non-digital methods.

Probably the most common example of remote sensing is an aerial photograph but there are probably hundreds of applications related to remote sensing ranging from space-borne satellites to under-ground geophysical systems. It has become a major component in the evolving Geomatics industry. In order to generate maps for GIS, most remote sensing systems expect to convert a photograph or other data item to actual measurable distance on the surface. However, this almost always depends on the precision of the instrument that is being used to capture the data. For example, distortion in an aerial photographic lens can cause severe distortions when photographs are used to measure ground distances. Using sophisticated software like PCI OrthoEngine can convert the photograph into an ortho photo which can be used to measure ground distances.

In order to coordinate a series of observations, most sensing systems need to know where they are, what time it is, and the rotation and orientation of the instrument. High-end instruments now often use positional information from satellite navigation systems. The rotation and orientation is often provided within a degree or two with electronic compasses.

The resolution determines how many pixels are available in measurement, but more importantly, higher resolutions are more informative, giving more data about more points. However, large amounts of high resolution data can clog a storage or transmission system with useless data, when a few low resolution images might be a better use of the system.

Like I mentioned earlier examples of remote sensing are very numerous. I have over the past decade and have used the many projects that I have been involved with along with actual examples of my work to help illustrate the principals of the various topics covered on the web site. I have included basic overviews for each along with images, presentations, papers and links to other related resources.

Examples of Remote Sensing

Remote Sensing Links

Digital Terrain Modeling – Aspect models

Real world example of slope and aspect

Aspect is measured in degrees (similar to a compass bearing) clockwise from magnetic north.In digital terrain modeling the Aspect of a surface refers to the direction (azimuth) to which a slope face is orientated. The aspect or orientation of a slope can produce very significant influences on it, so it is important to know the aspect of the plane as well as the slope. Together the slope combined with the aspect of the surface can virtually define the surface plane completely in digital terrain modeling.

Aspect is measured in degrees (similar to a compass bearing) clockwise from magnetic north. A surface with 0 degrees Aspect would represent a north direction, an east facing slope would be 90 degrees, a south facing slope would be 180 degrees and a west facing slope would be 270 degrees.

Aspect map derived from a digital elevation model of Lismore, Nova Scotia

The example shown to the left (for larger image click here) is a raster aspect model of Lismore, Nova Scotia was derived from a digital elevation model (DEM) calculated using PCI Geomatica remote sensing software. It is represented with a grey scale color ramp and helps to indicate what direction slope faces are orientated.

The image above is of an actual bedrock cliff with some technical information embedded onto the image to help better understand slope and aspect relationships. The black arrow represents the slope or the measured angle that the rock is dipping towards.

The aspect is the orientation that the arrow (slope) is pointing with respect to North, therefore the aspect for this slope would be in an easterly direction and often represented by 90 degrees. The blue arrows represent the X, Y and Z dimensions that the combination of both the slope and aspect would use to represent the terrain features.

Example of an Aspect Map

The image below is an Aspect Model that I derived from a digital elevation model (DEM) of Lismore, Nova Scotia. The aspect values of the slopes of the DEM are represented in the model by a 0-255 grey scale color ramp. Click here to learn a little more about Aspect Models and how the image below was created.

Aspect map derived from a digital elevation model of Lismore, Nova Scotia

Slope

image of a cliff demonstrating Slope calculations

The slope or the gradient of a straight line within a Cartesian coordinate system is known as the measure of how steep a line is relative to the horizontal axis.

In calculations; it is generally represented by the letter m, and defined as the change in the Y coordinate divided by the corresponding change in the X coordinate, between two distinct points on the line (X1, Y1 and X2, Y2). Since the Y axis is vertical and the X axis is horizontal by convention, slope is often referred to as the rise over the run or the change in the vertical coordinates, divided by the change in the horizontal coordinates.

Basically, the larger the slope value, the steeper the line is. A horizontal line has a slope of 0, a 45 degree line has a slope of 1, and the slope of a vertical line is typically undefined. In trigonometry two lines are considered to be parallel if and only if their slopes are equal or if they both are vertical and therefore undefined. Two lines are considered to be perpendicular if and only if the product of their slopes is -1 or one has a slope of 0 and the other is vertical and undefined.

There are two common ways to describe slope. One method is to use the angle of the slope in degrees (0 to 90), and the other is to represent the slope as a percentage (0 to 100). Expressing slope as a percent is common but can be confusing because a percent slope can be greater then 100%. A 100% slope is actually only a 45 degree angle due to the fact that the rise and run of a 45 degree angle are equal and when divided always equals 1 and when multiplied by 100 will equal 100%.

Slope Model / Map for Lismore, Nova ScotiaIn terrain modeling we generally model an entire surface and not just one line so we need to calculate the slope of a best fit surface plane (which is made of lines). Because the terrain model is usually continuous across the entire surface, it is important to be able to calculate how to represent grid cells (or pixels) when going from one elevation to the next. To do this we generally need to know the aspect or the direction that the surface plane is sloped as well. Together the slope combined with the aspect of the surface can virtually define the surface plane completely.

In the example shown to the left, a slope map of Lismore, Nova Scotia was derived from a digital elevation model (DEM) calculated using PCI Geomatica remote sensing software. It is represented with a grey scale color ramp therefore the color white represents a 0 slope and the shades of grey increase through to black which represents an undefined slope. The majority of slopes for this map do not exceed 17 degrees (except for vertical slopes) as this is a relative low lying area of Appalachian terrain.

The image above and to the right is of an actual bedrock cliff with some technical information embedded onto it so it may be used to help better understand slope. The black arrow represents the slope or the measured angle that the rock is dipping towards. The slope in the image would be 45 degrees approximately so the slope would be 1 or 100%. The rise and the run of a slope with a 45 degree angle will always equals 1, thus when multiplied by 100 to calculate percent slope will equal always equal 100%.

Example of a Slope Map

The image below is a Slope Model that I derived from a digital elevation model (DEM) of Lismore, Nova Scotia. The values of the slopes of the DEM are represented by a 0-255 grey scale color ramp, therefore the color white represents a 0 slope and the shades of grey increase through to black which represents an undefined slope. The majority of slopes for this map do not exceed 17 degrees (except for vertical slopes) as this is a relative low lying area of Appalachian terrain.

Click here to learn a little more about Slope Models and how the image below was created.

 

Slope Model / Map for Lismore, Nova Scotia

Kejimkujik National Park and Historic Site ortho photos

Below is a poster created showing a seamless ortho photo mosaic of Jeremys Bay Campground, Kejimkujik National Park and Historic Site created from several 25 cm resolution aerial photos flown while studying at COGS in 2003. The image has the location of campsites layered on top and was created for a Spatial GIS Vegetation Database and GIS Spatial Modeling project that I worked on during the summer of 2004.

25 cm aerial photos of Jeremys Bay Campground, Kejimkujik National Park and Historic Site

3D Perspective View Samples

Port Lorne, Nova Scotia

[*image source: PaulIllsley.com]

The following images are examples of perspective view models that were generated from a 2 meter LIDAR DEM integrated with 50 cm digital orthophotography of Port Lorne, Nova Scotia. Port Lorne is a coastal community along the Bay of Fundy with a relatively steep terrain so the images represent different perspective views depending on the source of origin and the direction that they are facing.

3D Perspective Views of Port Lorne, Nova Scotia

The colored arrows on the key image to the right represent the different perspective views that were generated from different locations and viewed along different directions (displayed in the images below). This image of the area also represents the traditional GIS view from above, as discussed above.

The first image below represents a perspective view (red arrow) of the area if it were viewed from the center of the image, above the wharf looking in an easterly direction.

3-D perspective view model of Port Lorne; Nova Scotia

The next two images below represents a perspective view (blue arrow) of the area if it were viewed from the upper right hand corner of the original image in a south west direction. The images that demonstrate the same perspective but have different types of LIDAR DEMs integrated with them allowing them to portray different data within them.

3-D perspective view model of Port Lorne; Nova Scotia3-D perspective view model of Port Lorne; Nova Scotia

 

The next two images (first one represented with the green arrow, second one in blue) represents a perspective view of the area originated in the upper left hand corner of the image, but in slightly different directions.

3D Perspective Views of Port Lorne, Nova Scotia

3D Perspective Views of Port Lorne, Nova Scotia

More info on 3D Perspective Views

GIS Mobile Mapping with ArcPad

Included here is a report written for an ArcPad / Trimble mobile mapping project, a presentation that covered all aspects of the project and more general information and links about GIS mobile mapping. The report includes all code used in designing the ArcPad application (visual basic, XML etc). The presentation was presented at COGS in Lawerncetown, Nova Scotia during the fall of 2003. The existing AGRG weather station network now consists of 14 tripods and 1 tower setups (as of Aug 2004).