Scaling with a reference point

It has been a while since I posted in here. I don't like when I have to post something that I have done before and know how to do, but it seems this time, I cant figure it out no matter what I do.

I am trying to scale something by using a reference point or if I know what the size should be. Let me explain.

If I insert a JPG into a drawing of a subdivision and want to make some changes. I would need to scale the drawing up to actual size 1:1

Now what I need to do and cant figure out is this. If the roads are say 6" now on the JPG, and I know they are to be 12'.

I know that at sometime, you add or type in the 12' as the finish size you want it to be, but no matter what I do or try, I cant seem to get it right.

Can someone help me with this.

Thank You.

If you know that something in the picture is in fact 6 inches and should be 12 feet, then SCALE by 24. 144(12 ft) / 6 = 24. Of course photo/jpg's being what they are, there is still a fair sized margin for error.

I am not talking about what the scale factor is. X2 or .5 or anything like that. I know there is a way, that you move the pointer to the base point, then type in the "R" for reference point. You then add the size you what the new size or scale to be.

I know I am probably not explaining it the best way.

You select the entities, then select the base point. Then it says Reference or specify scale factor. Type "R" then specify reference length. This is where I get stuck and it does not give me the size or scale I want. I have done this before, but I have tried everything and cant seem to get it again.

I am not talking about what the scale factor is. X2 or .5 or anything like that. I know there is a way, that you move the pointer to the base point, then type in the "R" for reference point. You then add the size you what the new size or scale to be.

I know I am probably not explaining it the best way.

 

 

You select the entities, then select the base point. Then it says Reference or specify scale factor. Type "R" then specify reference length. This is where I get stuck and it does not give me the size or scale I want. I have done this before, but I have tried everything and cant seem to get it again.

It is not really scaling with a reference point, but rather with a reference length,

which is defined by the selection of two points, or can be entered numerically.

After typing R, to select REFERENCE scaling, and specifying the basepoint,

you need to define (SPECIFY REFERENCE LENGTH) in one of those 2 ways.

Subsequently all selected objects will be scaled accordingly, to your NEW LENGTH (numerically entered at the commandline).

Failing that, the software has no reference, from which to calculate the scaling operation.

Since you know that the road is shown as 6" wide in your jpg,

you can enter as your reference length, then enter as your new length,

and the software will do the math for you.

I hope that helps you.

If you have a known existing length, and a known proposed length, there is no point in making it as complicated as the reference option. Select, "SC" "ENTER" "click basepoint" "24" "ENTER", done. Again, scaling up images is at best, only an approximation, unless they are actual size reproductions of FLAT documents or drawings. I included the scale factoring for the noobs.

If you have a known existing length, and a known proposed length, there is no point in making it as complicated as the reference option. Select, "SC" "ENTER" "click basepoint" "24" "ENTER", done. Again, scaling up images is at best, only an approximation, unless they are actual size reproductions of FLAT documents or drawings. I included the scale factoring for the noobs.

When measuring or scaling anything by "eyeballing" accuracy increases exponentially as the relative distance between the points increases. If you measure or pick two reference points that are only 6" inches apart and you're off by even one pixel, you'll be off by at least 24 pixels after you scale up 24x. IMO, when "eyeballing" measurements or reference points, the most accurate method is to scale twice Once to get it "close" to the actual size, then again to refine the size/scale where eyeballing errors won't be unnecessarily compounded by altering the overall scale.

Thanks yall. I was forgetting to input the length of what I was wanting to scale. In this example the 6". Then I put in the length I wanted it to be, 12'. So thank yall for the help. That is what I needing.

So thank ya Dana, DadGad, and nestly.

When measuring or scaling anything by "eyeballing" accuracy increases exponentially as the relative distance between the points increases. If you measure or pick two reference points that are only 6" inches apart and you're off by even one pixel, you'll be off by at least 24 pixels after you scale up 24x. IMO, when "eyeballing" measurements or reference points, the most accurate method is to scale twice Once to get it "close" to the actual size, then again to refine the size/scale where eyeballing errors won't be unnecessarily compounded by altering the overall scale.

If you know you're off by 0.99 or 0.98, it means you "eyeballed" a couple of points to obtain a measurement, right? Then calculated the difference between the "eyeballed" measurement and the desired measurement. I guess my question is that since you have to "eyeball" measure to know how far you're off, why not just use the [R]eference option and skip extra steps and the rounding errors? (ie it's never going to be exactly 0.99 or 0.98 it's going to be something like 0.9845218642214564)

If you know you're off by 0.99 or 0.98, it means you "eyeballed" a couple of points to obtain a measurement, right? Then calculated the difference between the "eyeballed" measurement and the desired measurement. I guess my question is that since you have to "eyeball" measure to know how far you're off, why not just use the [R]eference option and skip extra steps and the rounding errors? (ie it's never going to be exactly 0.99 or 0.98 it's going to be something like 0.9845218642214564)

Cadlee was the original jpg a aerial image as you talk about a subdivision if so with this type you can get a georeferenced JPG when produced you get an extra file a JGW this hold the scale factor and location values, we do this everyday, we have a rescale image lisp that reads the jpg and the JGW and rescales etc I got the program from here I think. Let me know if thats what you want.

I do this all the time. For scale reference as you have shown where 6" needs to egual 12'. Scale command, pick jpeg, pick basepoint, select "r" for reference first number 0.5 second number 12. This is not all that accurate because it is almost impossible to get the first dimension exactly 6" so here is the best way. Set the units to decimal, 8 decimal places, draw a pline along a known length. List the pline to get actual length to 8 decimal places. Copy the listed length from the command line and start the scale command, select all the objects. Pick base point, select r for reference, for first dimension paste the copied listed length, for the second dimension enter the known distance in inches. This will provide you with a very accurate dimension although still not perfect but much closer. Sounds more complicated than it actually is and doesn't take long to get used to, I've taught it many times to my new drafters. I usually trace the entire drawing before I do the scaling but that is more of a preference.

I do this all the time. For scale reference as you have shown where 6" needs to egual 12'. Scale command, pick jpeg, pick basepoint, select "r" for reference first number 0.5 second number 12. This is not all that accurate because it is almost impossible to get the first dimension exactly 6".

Can you explain why?

You are typing a number. AutoCAD knows what that number is. I would think that there would be no chance for an error using this method.

Can you explain why?

 

 

You are typing a number. AutoCAD knows what that number is. I would think that there would be no chance for an error using this method.

The inaccuracy comes from the insertion of the jpg or pdf. Unless you know a way to insert it to scale which if so please let me know. Because of the inaccuracy of the scale of the jpeg, drawing/tracing a line or pline along a known length and with 8 decimal places you may get that the 6" is actually (ex. 5.87534221) using this length as the first number and 144 as the second the accuracy is increased. Depending on what type of drawings you are doing this is the most accurate method I have found. I have to do this much more than I like and am about to do the same with a large pdf and tracing of contours where finding a reference known length will be difficult. For me accuracy with AutoCAD is critical and while going from 6" to 12' if 12' is your largest dimension the accuracy may not need to be as high but I often have much greater distances and need to be as close as possible. Hope this explains why.

Okay, that wasn't clear in your post. I agree. Not sure that you need to set the units, though. I believe that is for other purposes. Even if your units are set to zero decimal places, you can still draw a line to any length. AutoCAD does not change the length of the line to round it off to the accuracy that you have in units. You can still manually draw to or put in as many decimal places as you like. The rounding off only takes place when reporting the length. Try it. Changing the accuracy does not change the actual length of the line.

Correct that autocad doesn't change the length of the line but the accuracy of the listing of the length is determined by the number of decimal places hence the changing of units. If you have the units set to 0.0 when listing you will only get 1 decimal place of accuracy if set to 0.0000000 you will get 8 decimals of accuracy and the length of your traced line will be known to 8 decimal accuracy. The line is traced to an unknown length so using the list command while units are set to decimal and 8 decimal places will give the most accurate length.

In the context of this thread, scaling with reference, changing the units is not necessary, unless you are using the reported length to manually enter the numbers.

In the context of this thread, scaling with reference, changing the units is not necessary, unless you are using the reported length to manually enter the numbers.

I had specified that for greater accuracy to follow the procedures I laid out. The thread was about the scaling of a jpeg to fit a known distance and in order to do so a changing of the units is required for greater accuracy. This did not apply to the initial thread which I answered by stating that entering .5 as the first number and 12 as the second would achieve the stated goal.

Okay, sorry, I did not read your procedure very carefully as I do almost the same thing but instead of using the listed length, I pick points. I thought that is what you said. Sorry for the confusion.

Okay, sorry, I did not read your procedure very carefully as I do almost the same thing but instead of using the listed length, I pick points. I thought that is what you said. Sorry for the confusion.

No problem. Sometimes trying to explain steps that I can just about do in my sleep doesn't always come out the way I mean. Been doing autocad for 27 years now and there are some new features and different approaches that I haven't had the time to learn. Been doing this for so long and still learn something all the time.