Orthos

Document No:  D12#434486
 
Revision:  4A
 
Date amended:  14-May-2015
The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Nathan Miller by e-mail or on ph (08) 9323-4669.

To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.

 

Revision Register

 

Ed/Version Number Clause Number Description of Revision Date
ISSUE 1 ALL  GUIDELINE DEVELOPED  10/06/03 
ISSUE 2 ALL  PAGE NAME CHANGED 1/06/03 
​ISSUE 3 ​ALL ​Content Cleanup ​01/02/2013
​ISSUE 4 ​Header ​Document Number Change ​16/06/2014
​ISSUE 4A​Header​Guideline introduction amended​14/05/2015

Table of Content


1 General

An ortho rectified  photograph is a photo that has been altered to show features in their true position on the earth.  Ortho rectified photos have had image displacement removed and corrections performed to create a picture which is equivalent to a map.  Ortho photos have true scale and a georeferenced position and are therefore ideal to overlay other digital georeferenced data sets such as design, cadastral, environmental information etc for planning and display purposes. Main Roads have developed a standard (67-08-12 Digital Rectified Images) for the production of ortho photos to ensure continuity and quality of the final product.

 

ortho.JPG

 

2 Where to find existing Main Roads Ortho rectified Photographs?

A small number of project specific ortho rectified photos have been created and are available to Main Roads personnel through the photographic library within Main Roads Survey and Mapping section.   Indexes to this library can be obtained through the Senior Mapping Surveyor. 

 

3 SkyView Imagery

The Department of Land Administration (DOLA) have created ortho rectified photos of the metropolitan area and other more populated rural areas.  A selection of DOLA SkyView images are available on line to Main Roads employees at no charge.   Access to SkyView images can be made through ERViewer software located on all Main Roads workstations.  Access to imagery can be gained by contacting Haydn Bufton (Data Manager.)  Meta data for all SkyView images held by Main Roads can also be found here.

 

4 Advantages and disadvantages of Ortho rectified Photographs

 

The advantages of ortho photos are;
  • Distance, angles and areas can be measured directly from the ortho photos.
  • Digital information can be overlayed over the image and will sit in its true position eg cadastral boundaries, designs, land requirements etc.  Overlays will be subject to ortho accuracies.
  • The image and information can be coordinated to a project zone.
  • The image can be printed at varying scales.
  • Copies of ortho photos are relatively cheap and quick to produce once the ortho photo has been created.
  • Specified areas of interest can be enlarged and cut out to be displayed separately.

The disadvantage of ortho photos is:

  • Relatively high cost for production in relation to mosaics.
  • Cost to capture new photography or use of outdated photography.
  • Production time due to processing and ground survey requirements.
  • Limitations on enlargements dependant on source data.


5 How to obtain new Ortho rectified Photographs

Quotes for the creation of ortho rectified photos can be requested from consultants by altering the sample fax to suit your requirements.  Before altering the sample facsimile you must address the following issues. The Main Roads standard for ortho photos must be adopted.  Refer to the 67-08-12 Digital Rectified Images standard or the Senior Mapping Surveyor. 

The final scale will address how much detail you will be able to determine on the ground.

Base scale of the original photographs will dictate the limit of final scales with reductions and enlargements.  

Age and availability of photography must be considered.

The size of the print must be taken into account when you are thinking about scales.  The final plot can become unmanageable when the size is too big. (i.e. greater than 2m)  Is the product for display or for a reference booklet?   Can the area be broken into sections if you need to maintain a large scale?

You must have a diagram indicating the area you require to be covered.  Preferably A4 in size for faxing purposes.  This is used in collecting base information and providing to the consultant a clear definition of the area to be covered.  It will also help the consultants determine the final size of your plot.

What coordinate system do you want your ortho photo referenced to?  Primarily Main Roads local project zones to match designs are used but in some instances GDA is a better alternative.

Do you wish to overlay any information?  What format is the information in? (Microstation, Moss, AutoCAD etc)   The consultant must be informed of the format of all the overlays and if they are required to edit or extract information from these files.

What coordinate system are your overlay files in? (GDA, AGD or local grids) Will the files you supply require transformations to get them into the same grid.

Where are you going to access your overlay data from? 
Examples of overlay sources are;

  • Cadastral information - accessed through the Property Management Branch within Main Roads.
  • Contours - accessed from the Data Manager within Surveying and Geospatial Services
  • Metropolitan Region Scheme boundary - accessed from the Planning Branch
  • Designs/site boundaries etc - from project managers. 

The Senior Mapping Surveyor can provide advice on techniques or check any quotes to confirm that they are based on industry standard and are "value for money".