Grid to ground scaling in autocad---is this correct

[BURTONSNOW2002]

hello,

 

I have a question regarding grid to ground scaling. We collect data in the field in state plane coordinates, when I get the points into autocad this is where I get confused. I gain the correct scale factor to convert from grid to ground and when I scale the points do I do it at 0,0,0 or is it okay to just pick a point in space to scale it? Anyways we use corpscon to get the combined scale factor. If someone could explain the correct way to do this it would be helpful.

 

Thanks

[CyberAngel]

Welcome to the forum.

 

I'm not sure what you're asking. If your points are in state plane coordinates, you should be able to drop them into a drawing as is. To keep them in SPC, yes, you need to insert them at 0,0,0.

[BURTONSNOW2002]

I understand that part of it, but when I want to scale from state plane to surface do I scale it using the combined scale factor (for example .99985232) and use 0,0,0 for the base point to scale from?

[KJB]

You don't specify how you are importing your survey points into AutoCAD.

If using Civil 3D, Check out Transformation in the Drawing Settings.

[BURTONSNOW2002]

I am using autocad 2004 with visisoft to import the points.

[KJB]

Sorry. I never even heard of Visisoft.

[BURTONSNOW2002]

Visual Survey 2004 is what it is called.

[BIGAL]

Wouldn't you apply a scale factor from the middle as you move around particuarly over long distances the scale factor can change.

[eldon]

I thought that a scale factor was a mapping trick to draw the curved surface of the Earth onto a flat bit of paper. It varies as you go across the grid.

 

A coordinate is a coordinate, but a line on the map does not have the same length when you measure with a tape measure (or EDM ). The measured length has to be multiplied with the scale factor to agree with the line on the map.

 

Normally, I would take the scale factor of the mid point of the survey area, but it depends on how large the area is, and whether the area covers a larger area in the North South direction or the East West direction. Not a simple matter.

[Tyke]

The scale factor is not a trick it's a mathematical function that allows you to represent the surface of a sphere in a rectangular grid and as you move away from the origin of your grid the scale factor changes. Usually a Transverse Mercator Projection is used to achieve this.

 

Imagine the lines of longitude on the globe, the further north you go (in the northern hemisphere) the closer the lines of longitude become. In a rectangular grid you have to stretch the northern part of the spherical grid apart so that it fits into the rectangular grid. Different projections produce different effects, the TMP is the one used in most surveying situations. HTH. If I remember right Ordnance Survey in the UK have some good informative matter on this subject on their website.

[eldon]

I should have been more careful with my terminology in this learned company.

 

But nevertheless, the use of scale factor, which varies from the central meridian, allows the production of "flat" maps of the Earth's curved surface. It is only applicable to plane rectangular cartesian coordinate systems and not to geographical coordinates.

[Tyke]

I should have been more careful with my terminology in this learned company.

 

But nevertheless, the use of scale factor, which varies from the central meridian, allows the production of "flat" maps of the Earth's curved surface. It is only applicable to plane rectangular cartesian coordinate systems and not to geographical coordinates.

 

It was intended as general information for all, not aimed at anyone in particular :wink: and your comments are correct.

[eldon]

To get back to the OP's original query, I would have thought that if the data had been collected in state plane coordinates, then there would be no further scaling needed. If the data were not in the correct coordinate system, you would have to transform them, which is usually not a simple single scale factor.

 

Where you do need the scale factor is if you want to take distances from the map, and then measure them on the ground. Map distance = Ground Distance x Scale Factor (where the scale factor varies as to how far you are from the Central Meridian. In the UK it varies from 0.99960 to 1.00050)

[Tyke]

To get back to the OP's original query, I would have thought that if the data had been collected in state plane coordinates, then there would be no further scaling needed. If the data were not in the correct coordinate system, you would have to transform them, which is usually not a simple single scale factor.

 

That's right and you need special software to do such a transformation on a set of coordinates, in AutoCAD MAP to transform points you already have in AutoCAd you can use The Affine Transformation function or a seven parameter Helmert Transformation (not part of AutoCAD) if you have sufficient control points over which you can do the transformation.

[eldon]

Having re-read the first post (my thought process is not very fast), I think that I can realise what the OP is getting at.

 

It does not matter what point you scale about, because you would be setting up a local grid, where the grid origin would be near the area of interest. The coordinates would no longer be state plane coordinates.