A crackdown has been launched against developers who make wind turbines look smaller than their real size in planning applications.

Companies applying for ­permission to build wind farms are to be given new planning guidelines amid fears some councils are being tricked into giving them the go-ahead.

Revised Scottish Natural ­Heritage (SNH) guidelines update eight-year-old rules on how photomontages and drawings should be presented to help local authorities and other planning authorities assess the impact of the projects.

It specifies the inclusion of larger images that are easier for the public and decision-makers to use. It requires the production of images with an equivalent focal length of 75mm, increased from 50mm, and recommends new digital methods to make it easier for the public to view images online.

Viewpoints out to 12.5 miles should be illustrated, and there should be "zone of theoretical visibility" maps to show where a wind farm would be seen.

A method to verify that images have been presented correctly is also included.

Brendan Turvey, policy and advice manager for renewables at SNH, said: "The new guidance will deliver a significant improvement in the way wind farms are represented."

He said it would ensure images were easier to use and give a clearer impression of how the wind farm would sit in the landscape.

He added: "It will also make it easier to illustrate cumulative effects. No visualisation can ever represent exactly what the wind farm will look like, due to different weather conditions, lighting, and turbine movement. But we think this is as good as we can recommend using current methods and technology."

Visualisations are used by ­developers to support planning applications and help councils and the public to consider potential landscape and visual effects. These include maps, photo­montages and wireline drawings.

The new guidance is supported by the Scottish Government, ­Landscape Institute, Scottish Renewables, and Heads of Planning Scotland. It will be phased in over six months, reflecting the fact that many developers have already taken photos for applications about to be submitted. Those producing visualisations will also be given detailed training by SNH.

It now recommends that viewpoints out to 20km should be illustrated using the new method.

Alongside the publication of the new guidance, SNH will shortly commission further research on wind farm visualisations. This will test whether the new methodology has improved the quality and accessibility of visualisations, with a view to informing its further refinement in future.

Brendan Turvey, policy and advice manager for renewables at SNH, said: "The new guidance will deliver a significant improvement in the way wind farms are represented. It builds on our experience of assessing wind farms across Scotland. It will ensure images are easier to use and give a clearer impression of how the wind farm would sit in the landscape. It will also make it easier to illustrate cumulative effects.

"No visualisation can ever represent exactly what the wind farm will look like, due to different weather conditions, lighting and turbine movement. But we think this is as good as we can recommend using current methods and technology.

"The challenge was to design a tool that meets everyone's needs but wasn't too complex. The guidance is part of our efforts to improve the assessment of wind farm applications and help get the right developments in the right places."