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Mapaid AS

Tjernveien 6
1397 Nesøya, Norway
Tel: +47 95184926

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Picture quality digital aerial photography
Written by Mapaid. Published 9 December 2006.

Oblique digital aerial photography is revolutionizing the use of aerial photography in area management and planning. The product does though currently not have any agreed standards or specifications for picture quality or other parameters. The result is that often offers are only compared by cost/price and not by what they deliver.

This is as wrong as comparing an offer for a 1:50.000 map with an offer for a 1:250.000 map. There are a lot more details in the 1:50.000 map. This document has been made to provide potential customers with an understanding of digital pictures and aerial digital photography and especially oblique aerial digital photography as Mapaid does it – carpet photographed..

What is a digital picture ??
A digital picture consists of many thousands of picture points (pixels) with different color that are organized (put together) to form a digital picture, much in the same way the traditional photograph consisted of corns of silver nitrate. When the picture is projected on to a screen the colors will be displayed with varying intensity. Together they form a picture we can recognize.

What do we mean with ‘picture size’ ? ?

The size of a digital picture depends on the number of picture points (pixels) used. This is most often described using the mega pixel description, but this is simply the number of picture points vertically multiplied with the number of picture points horizontally. A picture that is described as 480*600 pixels is simply a picture that is composed of 480 picture points vertically and 600 picture points horizontally – or 0,28 mpixels.

The disc volume file size (kilobytes or megabytes) of a picture depends on the number of pixels.

A picture that is 17 MB large on your disc will contain 33% less pixels than a picture that is 22 MB. You can compare this to regular (old fashioned) pictures taken with a silver corn technology – the finer the corns, the better the resolution and the better the pictures can be enlarged but there are a lot more corns..

What is most important for digital picture technology?
The most important parameter for digital photography, in the same way as for the old fashioned photography, is the size of the picture, i.e. the more pixels the better. The more pixels you get the more details you can see and the better you will be able to enlarge the picture. This is the detail resolution of the picture. A number parameter for this is to divide the width of the picture (in meters/physical real world values) by the number of pixels in the width of the picture:

Detail resolution = Width of motive/Number of pixels in the width

Example 1: Motive is 500 meters wide in the picture. The number of pixels in the width dimension of the picture is 3000, and then the detailed resolution is 16 cm (500m/3000 pixels).

Example 2: Motive is 250 meters wide and the number of pixels in the pictures is 2000*3000. Then the detailed resolution of this picture is: 250 meters/3000 = 8 cm).

So two pictures consisting of the same number of pixels can have very different resolution and their use can be very different.

How to achieve optimal coverage of an area?
It is more reasonable (costs less) to cover a given area with less pictures, but this may not always be the best solution. With digital oblique aerial photography in hilly regions or in cities and built up areas ‘blind zones’ can occur if the resolution is too low. One should also define an ideal overlap between pictures and/or define that one wants the areas photographed from several directions.

Our experience shows that the overlap should be 40-60% both in picture high and in the width. This is also to ensure that each and every area photographed is within the ideal photographic distance and has a sufficient detail resolution. The balance between cost/price and resolution, coverage, blind zones and overlap is important in defining the usefulness of the pictures.

What do we mean with ‘working area’ ?
Oblique pictures will have less detail resolution the higher up the picture you look because the distance to the area photographed is larger. It is therefore not advisable to use the full picture coverage as the working area. We recommend only using the lower part of the picture where the area is closest. Furthermore buildings that are along the side of the picture may be better covered by adjoining pictures. A pictures work area is a combination of several of these factors and also depends on the application for which the picture set is intended. It therefore requires some experience and good communication between Mapaid and the customer to decide on the optimal solution and set up for the picture set.

More and more GIS software allows the user to add coverage polygons. It is obvious that if one adds the whole picture then the user will get a large amount/number of erroneous pictures covering a single area. It is the work area that is relevant to add as a coverage polygon.

It is therefore critical to use the correct work area and overlap parameters when defining how a coverage polygon in GIS shall be defined.

What does the number of colors in a picture mean?
Whereas the pixels in the pictures on your television screen only consist of the three primary colors a pixel in a digital photograph can have millions of colors.

The human eye is capable of discerning very small color differences (nuances). The more colors a picture has the more we can see details. Color depth is measured in bits and a 32 bit picture has better color depth than a 24 bit picture. It is the original picture (the photography) that determines the color depth. Color depth will also vary with light intensity and resolution, so there again it is very important not to choose combinations that are not useful and/or workable.

Picture quality also depends on file type and packing
From the original picture and all the way until the final storage of the picture to be supplied there is a chain of decisions to be made that will significantly influence the quality of the product you receive as a customer. This chain is a multiplicative chain meaning that the weakest link determines the overall quality. The most usual storage format for digital pictures is ‘jpg files’. A jpg file can be packed (compressed) to different sizes. The more you compress the smaller the picture is, but you also loose resolution and picture quality. Mapaid recommends using the high quality original formats as the base storage and then reducing picture size for work. This way the customer has available all the data that was initially in the picture.

What other services should accompany an aerial photographed product?
If you only use a few, less than 20, pictures then you can manage to keep track of them. But when the number of pictures exceeds 100 then it becomes necessary to think about a sort and search utility that can assist you in finding the right pictures you need for a specific task. To integrate a picture data base in to a GIS system you will need a data base to accompany the pictures that follows ESRI Shape protocol. The data base file should contain the geographic coordinates for the pictures ideal centre point (centre of gravity) and preferable also the geographic coordinates for the corners. In addition technical details about the pictures parameters, such as flight hight, date, etc., etc., should be stored and be easily retrievable.

Who can use the pictures?
A lot more people than you think can use the pictures. Most organizations only have one or two GIS licenses as these are expensive. Adding in a simple easy-to-use search utility that allows the user to search by address or other utility numbers will allow many more to gain access to the data base of pictures at a very useful level.

The right to use the pictures
What rights do you have in your agreement to use the pictures. Aerial photographs are like other intellectual property and therefore you need to have explicitly stated for what use the pictures are leased/rented or purchased and/or if you e.g. are allowed to sell them or not.

Installation qualification
Since digital pictures are difficult to quality test you need to make sure that you e.g. get demo pictures with the agreement and that you can compare the pictures you buy with these with respect to the quality parameters that have been defined in the agreement. And make sure you do this immediately when you receive the pictures.

A Quality Specification for Aerial Photographed Picures can look like this:


Purpose:	Describe for what purpose the pictures are being ordered.
Coverage area:	Define the total area of coverage (square kilometers, area names and use a map)..
Detailed resolution:	Define the detailed resolution in cm.
Overlap/Direction:	Define the degree of overlapp (Horisontal/Vertical) and if there should be pictures taken from multiple directions in specified areas.
Width of motive:	Number of meters width for rach picture.
Number of pictures:	Define the number of pictures expected.
Picture size:	Define the size(s) of the pictures (Pixels).
File size:	Define file size(s)
Original medium:	Digital camera or regular film.
Color depth:	Number of bits color depth and number of colors in the delivered pictures and in the original material..
Additional services:	Define the additional geographic and technical data and information that should accompany each picture and how. . Software for address and other searchable data Licensing conditions for the software. Help to install and get the picture base operational.
References:	List of References.
Samples:	Use these to have reference pictures to compare the supplied product with.

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