dimmension in a dwg

hi im new as you will see,so i will try to make myself understood,first problem is when a dwg has been created in scale 1;200 the size of the road is ok but as another section from another dwg has been added it is not same scale,so when using measure to accuraltly set a distance it represents a very large dim,the dwg looks ok but dim are wrong.eg.road is accuralty 8m wide but in dwg it is 146m wide.can anybody help.cheers

Always draw everything at full size 1:1 and you won't have this problem.

The best principle is to draw everything 1:1 and if you need the drawings at different scale, even different scale in the same sheet, you can use paper space. The paper space itself should be printed in 1:1. It contains viewports (containing model views of your drawings at set scale).

For the dimensions and annotations: you can do it in paperspace or in modelspace. The scale will not affect the accuracy of the dimensions.

One problem that you can encounter is that you cannot use dimensions to span across two viewports (this is the case with long sections with lots of details at the ends). In such case, there are but two options: 1. apply the dimension in the paperspace and change the dimensions manually, 2. apply the dimension in modelspace where you will have the accurate dimension but you may need to realigned the text so that it does appear in the paperspace.