I often get requests for aerial photographs. Where can I get the latest aerial photos for free? How can I get the same image as the ones that Google uses? Why are Google's images so old?

Aerial photography is a standardized industry and taking those photos (either by satellite or air plane) takes time and money. There are literally thousands of companies that capture images of our earth from a remote location. Aerial photos, satellite images and other such products are the result of what the industry calls remote sensing.

Remote sensing is the act of collecting information from an object while not being in direct physical contact with that object. Remote sensors (the equipment gathering the information from the remote object) are varied and complicated. Different sensors can gather different wave lengths of the electromagnetic spectrum so that you end up with very different image products, so that in turn can be used in a variety of applications. For example, near infrared images are often used to classify on image into vegetation types because the of the nature of how light passes through and is reflected by vegetation is its various stages. You can read more on the topic at the NASA Remote Sensing Tutorial page. You can read to your hearts content about TM Imagery, SPOT and plain old aerial photography.

However, most folks just want a natural color image for a map base. Depending on the final application, resolution, date and quality of the image (no haze or smoke) are the most important features folks are interested in.

Resolution

The resolution of aerial photos (or satellite imagery) is really dependent on one main factor during which the images were taken: Height (or distance) the remote sensor is from the surface of the earth. Regardless of how the final product is eventually sized with enlargement or reduction (or zooming in or out), the original scale at which the photo was taken will determine what sort of features you will be able to distinguish (i.e., large rivers vs small streams). So, when shopping around for aerial photos, find out what the original resolution or scale of the aerial photo is. For example, if you are hoping to distinguish individual homes and trees in your neighborhood, you will most likely need an image with a pixel resolution of under 1 meter (or a scale of 1:10,000 or greater).

Date

This is an obvious one, when was the photo taken? Are you looking to compare different years, highlight development or a change in vegetation?

Quality of Image

The quality of an image depends on the type of remote sensor used and weather conditions. Unless you really know what you are looking for, you don't have much control of the type of sensor used, however, you can look for images that were taken in summer. Why summer? In North American, the clearest days often occur during the summer months. There are exceptions, of course, however, you will be hard pressed to find any aerial photography taken during other times of the year unless they were taken for a specific reason. Prior to purchasing any aerial photography, ask if the images are haze and smoke free

So what about free photos and the ones that Google uses?

The USGS has a library of Digital Orthophoto Quadrangles (DOQ). These can be accessed from a variety of places and are entirely free! We pay our taxes from some reason right? Well, here's one of 'em. They usually come in TIFF format, are geo-referenced, and are usually about 1m in resolution. However, they are black/white photos. In addition, they are not often updated. Another government source (read - free) are the images from the National Agriculture Imagery Program (NAIP). The resolution on these are at 1m as well but they are natural color (with some available in near IR for some areas). These are also available from a variety of sources. Another issue with these as they for taken for agricultural purposes, they do not cover cities or urban areas.

As far as the images that Google uses, have you noticed on the right hand, bottom side of the image there is usually a reference to the image originator, like DigitalGlobe, Sanborn or TerraMetrics. These guys want money for those images! And Google, being inexhaustibly rich, can pay for an entire world's worth of images. Keep in mind, however, that these images are not real-time (imagine the cost!) but they are usually the best (clearest and latest) available for a given area from their image sources. There are, of course, other image sources. There are thousands of local aerial image providers throughout the country and I am sure there is one near or in your town! You can look them up in your phone book under Aerial Photography, Photographers Aerial or Surveyors Aerial. Here in Truckee, CA, we have GeoCADD. I'm sure there are others but that is who I use. In addition, there are online sources like Topozone, Terraserver and GlobeXplorer.

Hope this was useful!