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### scale and earth curvature and refraction

hello everybody,

i would like to know

1. what scale factor in total station and how it will take for all measurements.

2. what is curvature and refraction of earth. and we eliminate all errors of measurements.

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#### Replies

In making viewshed calculations, the earth's curve and the refraction of light in air have an impact (albeit minor) on lines of sight. I this post, I provide some links describing how Arc calculates both variables. I would love to find out how to recalculate the "refractivity coefficient" of 0.13 to other situations. I also put a plug in for fuzzy viewsheds, a drastic improvement that was first suggested 15 years ago, to be mostly ignored in practice until very recently.

Corrections for the Curvature of the Earth

ArcGIS 9.2 makes it much easier to correct for these two variables with standard corrections, but it is not clear how it makes these corrections, or how they might be modified. The help file is clear on how to modify the parameters of the analysis. At the bottom of this page, they explain calculations for the curvature of the earth.

How Viewshed Works

Another help file focuses on how curvature is calculated:

How Curvature Works

A more detailed technical article from ESRI elaborates on this:

Problem: Are there any more details on the outputs and calculations...

A very useful post (by someone who usually puts up great posts) on the ESRI support forums explains this as well, as he describes how to calculate curvature manually. This was before the days of Arc's default correction, which seems perfectly good for estimating a variable that usually has minor effects (objects 10 km away look about 7.8 m higher; however, the effects are quadratic, so at longer distances, this is a big deal).

Another Solution I found..

ArcGIS Desktop Help 9.2 - how viewshed works

what scale how to adjust it all measurements

Land Surveyor

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