Where to start? The process below should help you decide what kind of aerial imagery you need and how to acquire it. You’ve got a few choices of satellite and aerial orthophoto. Answering a few questions about your project can help you come to the right decision.
1. How large is the area of interest?
Area is closely tied to cost. Most imagery is sold in quantities related to the size of the geographic coverage, so large scale projects will need large scale budgets to match. In the case of a very large area, say, 100 km2 or more, the best answer will almost always be satellite imagery. The value for money for large areas compared to other image sources is unparalleled. For areas covering a few rural properties, however, aerial imagery taken from fixed wing aircraft is usually less expensive when purchased from existing archives.
2. How recent does it need to be?
You probably have good reasons to want the best and the newest available, but if your project is in a stable area that doesn’t really change from year to year, there’s nothing wrong with ordering archived imagery that’s a few years old. Archived aerial imagery is normally taken in early Spring after the snow has melted, before the trees are budding, and without cloud so that ground features are as visible as possible, so, older aerial imagery can actually work to your advantage in many cases. Satellite image archives are collected year round, allowing the chance to compare the same area over different days, seasons, or years…weather permitting.
For areas where development has taken place recently, it will usually be necessary to order new imagery. In that case, simply call for a quote. If you’re interested in canopies, crop health, or other natural features that have a specific time frame other than early Spring, it will also be necessary to order new imagery according to your specifications.
3. What are you going to do with it?
Base Layer or Spectral Analysis?
If you plan to simply use it to give context as a base layer to build a map on top of, then you’re looking for natural colour imagery. Any imagery provider will have this option. If you plan to use the imagery in any sort of spectral analysis or plan to process the imagery to produce a derivative image, then you’ll want multispectral imagery such as infrared which can come from some satellite imagery directly, or, aerial imagery on request. Be sure to tell the salesperson that you have these advanced requirements when placing an order for imagery since the default order is typically natural colour.
How much detail would you like to see in the imagery? If your area is large, then you may not need to zoom in on small details in high resolution imagery, which can be cumbersome to store and manage. The clarity of imagery is described as resolution in cm, meaning the coverage of a single pixel. Satellite imagery has resolutions normally between 40 cm and 60 cm, meaning a single 40 cm pixel covers 40 cm by 40 cm on the ground. Since a pixel is the smallest unit in the photo and only a single colour can be assigned to a pixel, no ground object smaller than the resolution will be distinguishable in the image. Colours of many small objects get averaged together. For example, in a 40cm image, a red and white beach ball measuring 30cm diameter in a grassy field will look like a single pinkish pixel surrounded by green pixels. Aerial imagery is of course much closer to the ground and can capture more detailed images, up to 3 cm per pixel. The same beach ball would take around 50 pixels, some solid red, some solid white, the round shape would be obvious, with a fuzzy transition from the ball to the grass. Will you be measuring beach balls? Probably not, but if you want to measure rooftop vents which are roughly the same size, you’ll need to spring for the high resolution option.
The resolution refers to the visible detail in the image, the precision refers to the geolocational error that describes how well the image will line up against your existing high quality data. Basically, how far off is any pixel in the image from the true location of that object on the ground? A photo supplier should be able to give you metadata to describe the qualities of the photo such as the resolution, the date it was taken, and so on including the precision, or error. The smaller the error number, the more precise the geo-referencing of the image.
If you require elevation data alone or in combination with photo, an image supplier will be the place to get it. The aerial imagery you most often encounter in the data marketplace has been orthorectified to correct for the movement of the plane, but the original unprocessed aerial imagery can be used to created digital elevation models. As the plane moves forward, the viewing angle of the camera changes slightly much the same way our eyes create 3D images by using two slightly different viewing angles. Similar but less detailed surface models can be produced from satellite imagery. The elevation data extracted from the photo are often offered as a complementary product when you purchase imagery, or, you can request the imagery supplier produce one for you at the time of purchase. If your area of interest is one that requires updated imagery, then you should request an updated elevation model be produced to match it.
Who will be the audience for the document you’re producing from the imagery? Check the end user license agreement and you’ll find that most imagery suppliers do not allow you to resell or redistribute the imagery without their permission. If you intend to use imagery for publishing, then you’d better consult with the supplier in advance to arrange for a special licensing agreement.
Satellite and aerial imagery archives are available for sale through various suppliers including First Base Solutions and can be ordered and delivered in a matter of minutes in some cases. Still not sure? We welcome your questions and invite you contact us at any time.