5. lecture 4 data capturing techniques - satellite and aerial images
- 1. L4 – Satellite and aerial images Cadastral Survey
- 2. Satellite Images A convenient way of collecting cadastral data is through satellite images. This is a comparatively new technique that makes it possible to use geo-referenced images and digitize features in the image directly on the screen Nowadays, satellite images with a resolution of less than one meter are possible to obtain in Ethiopia.
- 3. Satellite Image If reference points with known coordinates are obtained, they can be used to determine the position, scale and rotation of the image compared to the ground coordinate system. To calculate this, the reference points must be possible to identify with high accuracy and without doubt in the image. Geo-referencing will be done by software such as ArcGIS. Using the tools of software, it is then possible to draw the whole parcels from details that are marked in the image. Parcels drawn from images should therefore be regarded as a helpful tool to collect data, but should always be followed by a confirmation survey on the ground such as handheld GPS.
- 4. GeoEye1 4
- 5. Advantage of Satellite Image Generally the image gives a very good overview of the survey, making it easy to verify that all parcels and details in an area are measured. Making a parcel is easier in satellite image than with ground survey. It is easy to detect or identify in the image if e.g. a parcel border line is slightly bent, and digitize this with some extra points along the line. This is often missed in ground surveys, where the bent border is often only represented by a single line.
- 6. Disadvantage of Satellite Images The image is describing the landscape as it looked like when the image was taken. e.g. two years have passed since then, the landscape might have changed considerably by changed parcel shapes, new roads, cut down trees etc…Change Leads wrong digitizing of parcel borders between two parcels. Clouds might obscure parts of the image. Error due to height differences in the image. To georeferencing the image, it is needed to find details in the image that can be clearly identified in the field.
- 7. Disadvantage of satellite Image Digitized parcel borders between two parcels might be wrong for two reasons. – 1st , two farmers might use the same crops, making the border invisible in the image. – 2nd , one farmer might grow two different crops on the same parcel, making one parcel look like two different parcels in the image.
- 8. Sources of errors Errors in the geo-referencing optical errors in the camera used Errors in surveyed reference points High precision GPS or total stations are recommended for the survey of the ground points for Georef. In General satellite images are excellent tool to acquire a general planning map, but confirmation in the field of each parcel is necessary. It is also important to check the image accuracy regarding both the resolution and the orthorectification.
- 9. Aerial Images Aerial photogrammetry involves the use of photographs taken in a systematic manner from the air. Aerial photography is the taking of photographs of the ground from an elevated position. They are then controlled by land survey and measured by photogrammetric techniques. Accuracies achieved are comparable with those obtained by land survey and in many cases the work is carried out more economically.
- 11. Advantages of Aerial over Satellite Images The considerably lower height of the exposure gives a high resolution, often on decimeter level in the ground coordinate system. From a stereo pair of images, a 3D model can be created from the stereo perspective of the details in the images. This will automatically give an accurate terrain model for the rectification.
- 12. Advantage of Aerial image The amount of needed ground control points is much lower than the terrain model needed for satellite image rectification. It may be made for places that are inaccessible to ground soldiers. It provides a current pictorial view of the ground that no map can equal.
- 13. Disadvantage of Arerial Image The main Disadvantages of using Aerial images is: Detailed variations in the terrain features are not readily apparent without overlapping photography and a stereoscopic viewing instrument. Recent aerial image of the given area is very expensive. The method to capture the photograph is expensive.
- 14. Next Time Lecture 4 Cadastral Maps