| What's New at NETR? |  |  |  | | This is a free periodic newsletter aimed at keeping our customers and website visitors abreast of the latest products and services available from Nationwide Environmental Title Research, LLC including updates to NETROnline.com and HistoricAerials.com. We offer this information as part of our commitment to being a premier property information provider. |
| MapViewer 3 | We recently rolled out some significant updates to our map viewer. We have dubbed these updates "MapViewer 3", which represents the latest in our continuing effort to improve HistoricAerials.com and remain your favorite source for historic aerial photography.
The major focus of this update is map navigation. We agree with many of you that having to manually change between navigation tools (Pan, Zoom In, Zoom Out) is unnecessary and annoying. MapViewer 3 completely does away with the concept of navigation tools and works much more like many of the other maps around the internet. You can double click anywhere on the map to zoom into that point. MapViewer 3 also responds to mouse wheel events. If you have a mouse with a scroll wheel, you can use the scroll wheel to zoom in and out. Finally, MapViewer 3 introduces a new "Zoom" menu that allows you to jump directly to any zoom level you want at any time. We think these changes make navigating through our collection of imagery a much more efficient and pleasant experience.
MapViewer 3 also makes some subtle changes to the way the Survey Tools work. In the past, you could make some survey marks on the map and then inadvertently pan or zoom the map beneath them, making your survey marks irrelevant to the imagery they were drawn on. This situation could cause confusion and we wanted to find a way to make sure that survey marks always apply accurately to the imagery they are drawn on. MapViewer 3 introduces the concept of a "surveying mode". Any time you begin using the survey tools, the imagery you are marking up will become static. To end a surveying session and return to navigation mode, simply choose "Survey Tools Off" from the "Survey Tools" menu.
Finally, MapViewer 3 makes some changes to our watermarking scheme. We are in business to make money and to make money we have to sell imagery. Therefore, it becomes necessary for us to take some reasonable steps to discourage users from stealing imagery from the free viewer. Watermarking is a necessary evil and we recognize that the biplane watermark we used previously was overly intrusive and unattractive. We think the new scheme strikes a better balance between allowing users to browse our collection and preventing them from stealing imagery.
We think MapViewer 3 is a vast improvement and we hope you agree! | | | | From Camera to World Wide Web | By request from our readers and on our discussion forums January's newsletter included a very brief history of aerial photography and this month, also by request, we take a look at how aerial photos are processed for web delivery. This overview, like last month's, is a brief and simplified explanation of the process in the hopes it is interesting, understandable, and not too technical.
The first step in preparing aerial photos for web delivery is digitizing, or scanning, the photographic negatives. This requires a highly specialized scanner capable of delicately handling 9” x 9” rolls of negatives. These scanned photos are stored digitally, typically as a tiff file.
Next, HistoricAerials.com's Photogrammetrists begin producing geometrically corrected images (commonly called ortho's) from the scanned images. An ortho is an image that has been geometrically corrected (orthorectified) to adjust for the shape of the earth's surface, the angles of the camera relative to the earth's surface, and camera lens distortion.
Before a scanned image can be orthorectified a spatial relationship must be established between the scanned image and a reference image. The reference image is an orthorectified image of the same area as the scanned image. The reference image also has location information stored in it's file so coordinates can be determined for the location of any object in the image. Our photogrammetrists establish a spatial relationship between the scanned image and the reference image by identifying landmarks common to both images. Using software the photogrammetrists use a mouse to place corresponding points on the scanned and reference image for each common landmark they decide to use. Intersections of streets, driveways and sidewalks are often used as landmarks. The software records the location for points on the reference image along with file coordinates for the corresponding points on the scanned image.
Figure 1. Measuring Points. Reference image on left, scanned image on right.
The file coordinates for the scanned image are based on the location of the pixel where the point is placed. Without getting too in depth, scanned images are raster images. Raster images are based on a grid. Each cell in the grid represents a pixel. The location of each pixel within the grid defines its file coordinates.
After many points are collected the software establishes a spatial relationship between the reference and scanned images and the scanned image is ready to be orthorecitifed. In the orthorectification process, values of the pixels are rearranged to move objects closer together or farther apart. To see an example look at the change in the shape and size of the large shadow area in raw image and the same area in the orthorectified image (Figure 2). Also note the straight edge along the top of the raw image and the curved shape of the same are in the orthorectified image.
Figure 2. Raw image on left and orthorectified image of same area on the right. Note the change in the shape of the edges of the photo and the shape of the shaded area. Phoenix, AZ 1982.
Before I continue, let me address a couple of questions that may have come to mind.
1.If we are using an ortho to orthorectify the raw photo how did the ortho get orthorectified? (Chicken or the egg???) Somewhere along the line a different source such as a map was used to compare landmarks and produce the reference ortho.
2.The primary purpose of HistoricAerials.com is to observe changes in an area through time. If areas have changed, how do we compare common landmarks? (Great question!) This is one of the many talents of a great photogrammetrist. Streets are widened or two lane highways become eight lane freeways, farms become housing communities and so on. Fast growing areas or large differences in time between the reference and the scanned images also present challenges. Other obstacles are forests, swamps, mountains, rivers and lakes. The images below show an area of land in Franklin County, IL and the same area after a dam was constructed.
The above procedure is for one photo. Now move on to the next photo and repeat the process. The second photo overlaps the first photo and some of the same landmarks can be used. As we progress, a quilt of overlapping photos is produced. The blue and green “blob” in the upper left frame of Figure 1 are all the points collected (over 4000) for this dataset of 128 photos of White County, IL 1938. The software tries to make sense of the locations of all of these points while considering other parameters such as the height of the plane, focal length of the camera lens, angles of the camera relative to the ground, and the scan resolution when the photo film was digitized.
Often when dealing with older historic photography these parameters are not known or are incorrect. I'm sure there wasn't much forethought in regards to computer processing of aerial photos back in the 1940's... or 1930's... or 1920's, and much of this information has either been destroyed, lost, or is otherwise irretrievable. Fortunately we have developed methods for projects lacking this information. Some of the procedure are beyond the scope of this discussion. Let's just say our staff has devised some very creative solutions to complete some amazing work.
At this point our photogrammetrists have collected points on an entire dataset of photos (which can range from a few dozen to several hundred photos with thousands to tens of thousands of points) and refined the collected points so the software can understand and properly place the photos so they overlap seamlessly and accurately. Now they construct a mosaic of the photos using the best areas of each photo along with any necessary graphic improvements to create a single image of the entire project area. After several more steps the image has been run through quality control, prepared for delivery and transferred to it's final storage location. Next it is added to the aerial photo database along with source information and photo dates (when available) and released for web delivery. It can be a long and tedious task but the reward of seeing a project completed and our database continue to grow is well worth it.
I hope you found this informative and interesting. We enjoy your feedback so let us know how we are doing and what you would like to read about in the future. We would also like to know how and why you use HistoricAerials.com. Drop us a line and let us know.
What's New this month at HistoricAerials.com:
- FL – Hillsborough County 1982
- FL – Orange County 1969
- FL – Palm Beach County 1968-1969
- IA – 17 counties in the northwest 2007
- IA – all counties 2008
- MO - Clay County 1970
- NY – Orange County 1965
Pick of the Month – Palm Beach Gardens, FL
Would you like to submit a "Pick of the Month"? You can use the "Email" button above the right side of the HistoricAerials.com viewer to send the photo and any comments to newsletter@netronline.com
Have fun browsing and see you next month! |
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