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Determining the format of your Aerial Photos for Mapping

Overview

Some people have called us regards problems with image files that are huge, that can't even be viewed with Microsoft Paint, or slow down their computer. Sometimes the image is well beyond what is required, and has the effect of degrading the performance of the entire mapping system. We hope this document gives you an understanding of how to work out which image size is best for you...

PAM Mapping (including gpMapper) can import BITMAP (.bmp), JPEG (.jpg), TIFF/GeoTIFF (.tif), and ECW (.ecw) image files. Most aerial photography suppliers can provide any of these formats with ECW being the most common as it is already geo-referenced for simple importing and a smaller file size than TIFF files. Google Earth and other online maps can be saved as a JPEG and then geo-located using the image digitiser in PAM.

Image resolution

Resolution refers to the clarity of the scanned image when displayed on your computer screen. The higher the resolution the clearer the image.

When dealing with scanners and printers, the commonly accepted term is DPI or dots per inch. Because we are dealing with real world images, the correct term to start using is Metres Per Pixel (MPP). The more MPP an image has, then the less quality it will be. That is, an image where each pixel represents 1 square metre will show far more detail overall than an image where each pixel represents 8 square metre.

For example:

Have a look at the demo farm supplied with the program. If you zoom in on the aerial photo you can zoom so far that each individual pixel takes up the whole screen. If you get the measuring tool up, you will find that each pixel is approximately 14 metres wide. Thus this image is 14 MPP.

There are limitations on the size of images that you can use. It’s a bit like which comes first – the chicken or egg to arrive at what is the best image size for you. Firstly, we’ll discuss how the size of image is calculated, so that if you are dealing with a 3^rd party you can clearly explain what you want.

Width and Height:

Let’s say you farm is 5km east/west by 10km north/south. That’s 5000m x 10,000m. If you wanted an image which was accurate to 10 MPP, that would be 5000/10 x 10,000/10 or 500 x 1000 pixels big.

As we shall see choosing the width and height is largely dependant on the metres per pixel that you want to choose, as well as colour depth…

Colour Depth:

You may have heard people talk about "256 colour images", "16 million colour images"... The jargon for this is "Colour Depth"... Without getting too involved with the complexity of the binary system, you’ll have to take our word for it when we say that:

An image that can show up to 256 colours needs 1 byte to represent each pixel
An image that can show up to 65,536 colours needs 2 bytes to represent each pixel
An image that can show up to 16 million colours (photographic) needs 3 bytes to represent each pixel.

Choosing a colour depth is up to you. If possible, you might choose the format which is the best compromise between image colour accuracy and memory concerns. Most aerial photos can be quite adequately displayed with 256 colours, most probably with 65,536 colours and most definitely with 16 million colours. The trade off is the more colours you use, then the longer time to load from disk, the longer to takes to draw, and the more memory it consumes. Fairport recommends that you use 256 colours. The speed of loading is generally more acceptable than the lack of colour accuracy.

Image Size:

The size of an image is calculated by width x height x colour depth.    For each colour depth, our 500 x 1000 pixel image (at 10 MPP) consumes ..

256 Colours: 500 x 1000 x 1 = 500,000 bytes = approx. 0.5 Mb
65,536 Colours: 500 x 1000 x 2 = 1,000,000 bytes = approx. 1Mb
16 Million Colours: 500 x 1000 x 3 = 1,500,000 bytes = approx. 1.5Mb

Note: There are 1024 bytes to a Kilobyte (Kb) and 1,024,000 bytes to a Megabyte (Mb)

These images are relatively small, so lets say we went for 1 MPP for our 5 Km x 10 Km farm

256 Colors: 5000 x 10000 x 1 = approx. 50Mb
65,636 Colors: 5000 x 10000 x 2 = approx. 100Mb
16 Million Colors: 5000 x 10000 x 3 = approx. 150Mb

Image Detail:

Perhaps you want to examine the sample photo’s that came with the program. Will you actually want to zoom in to areas where a pixel is only a metre wide? If, for day to day usage, you’re quite zoomed out, a 15 MPP image may be more than enough. You could always register both a 2 MPP image and a 15 MPP image and put them on separate layers, making the layer containing the more accurate image visible when you need the accuracy. (Images aren’t actually loaded into memory until they need to be displayed)

Computer memory:

As you can see, images can easily get quite large if you go for greater image detail, and the actual size of the image may exceed the available memory in your computer. Whilst you have the detail, the performance of the mapping software will suffer as the computer strives to find enough memory to handle the aerial photo and have memory left over to do actual mapping functions.

For example, if your image is 80Mb and you only have 64Mb of available Ram, you will spend a lot of time waiting for the computer to respond. The total size of your image would want to be less than your total computer Ram capacity (remembering that all programs including Windows itself needs available Ram too).

Caveats:

We have discovered that some video cards on some machines can’t handle larger images. In particular, we wouldn’t recommend going over 2048 x 2048 pixels as your machine may not be up to the task.

What do you do if you have a huge property, that even at 15 MPP the image will exceed 2048x2048 pixels?! Easy, simply get multiple images that don’t exceed the size, or are of a more manageable size. The program will handle loading each image as required, so you will still be able to view them simultaneously. You will need coordinates for each image for the digitising process

In Summary:

    When ordering your scanned image, the rule(s) of thumb for scanning resolution are :

• Windows Bitmap file format (BMP), 256 colours
• Meters per pixel: It will depend on your computers memory and the size of your property. Using the rule of thumb calculation: Lets say that we use an image size of half your computer's RAM as an ideal size for the highest possible resolution image... The formula is this (assume we have 32 Megabytes of RAM so Half of 32 is 16) and we have our farm dimensions in Metres Accross and Metres Down:

  (Metres Across X Metres Down) Divided By (16 X 1,000,000) is approximately the Metres Per Pixel (MPP)

Or... If you want a certain MPP size, you'll need to work out the image size like this...

Image Size = (Metres Across X Metres Down) Divided By (MPP X 1,000,000)

If the image file is not already geo-refereneced then if possible ask the image supplier to write down for you two co-ordinates (we recommend the TOP-LEFT Corner and BOTTOM-RIGHT Corner geo-locations) of your property boundary and also supply a photocopy of the exact location of those co-ordinates. The co-ordinates can be in longitude and latitude or in "XY" (UTM) co-ordinates.

For Aerial Photos & Mapping Data in South Australia

Department for Environment and Heritage 
Level 1, 100 Pirie Street, Adelaide SA 5000
* Phone +61 8 8463 3999
* Fax +61 8 8463 3900
   http://www.mapland.sa.gov.au

For Aerial Photos & Mapping Data in Western Australia

Landgate (Western Australian Land Information Authority)  
1 Midland Square, Midland WA 6056
* Phone +61 9 9273 7341
* Fax +61 9 9273 7666
   http://www.landgate.wa.gov.au