photoshop matching scales and pixels

8/18/2019 PHOTOSHOP Matching Scales and Pixels

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Matching Scales and Pixels

There may be many occasions when you want to match the scale of an EPS file to an existing

raster image. This is a very common requirement when dealing with maps. You may have

drawn some information in AutoCAD and you then want to overlay it onto a raster plan or map

base in Photoshop. The illustrations below show a typical example. The results of a

topographical analysis created in AutoCAD are shown on the left and this data needs to be

overlaid on an Ordnance Survey 1:50,000 raster map tile in Photoshop, shown on the right.

Initially, this seems rather a conundrum but in fact, all we need to know is the map scale and the

resolution of the raster image. Since we know the map scale (in this case 1:50,000), all we need

to find is the resolution of the raster map. Even this is relatively straight forward.

We know that the grid lines on the raster map are 1km apart. We also know that the grid lines

on a printed 1:50,000 scale map are 2cm apart. All we need to know now is how may pixels

there are between the grid lines on the raster image. This can be achieved using the Measure

tool in Photoshop. If we measure the distance between grid lines, we find that there are

200 pixels per kilometer. This is consistent with the overall dimensions of Ordnance Survey

raster maps. The 1:50,000 data set comes in 20km x 20km tiles and each tile measures 4,000

pixels x 4,000 pixels. A quick calculation confirms our measurement. Now that we know this, we

can calculate the resolution. If there are 200 pixels per kilometer and each kilometer prints at

2cm, the resolution must be 100 pixels per centimeter. See the Ordnance Survey web site for

more information on the 1:50,000 raster map series.

http://www.ordnancesurvey.co.uk/






So, we must now print the

AutoCAD drawing to an EPS file at a scale of 1:50,000 and then open the resulting file in

Photoshop at a resolution of 100 pixels/cm.

In AutoCAD, plot the drawing to EPS, setting the scale to 1:50,000 or 1mm = 50000 drawing

units (assuming that your drawing units are in millimeters). In most cases the drawing unit used

for mapping data is the meter. If this is the case, your scale should be set to 1000mm = 50000

drawing units. Select an appropriate paper size and make sure that you preview the plot to

ensure you get what you expect when you open the EPS file.

Of course, all of the above assumes that you will be plotting from Model Space, which is

probably the simplest thing to do but you can plot from a Paper Space layout if you want to. In

this case, you will need to scale the view in the viewport rather than the plot.

In Photoshop, open the EPS

file you just created.File Open… from the pull-down menu. Select the EPS file and when you

see the Rasterize Generic EPS Format dialogue box, ensure that the Width and Height are

expressed in centimeters and that the Resolution is expressed in pixels/cm. Once you have

done this, you can set the resolution value to 100. The dialogue should now look like the one

shown on the above. Click the OK button to rasterise the file. This may take a couple of minutes

if the required pixel dimensions are large.



Once you have both the map base image and the imported EPS image in Photoshop, you can

use the layers palette to drag the EPS layer and drop it over the map base. Make sure that the

colour mode of the two images is the same otherwise nothing will happen. Ordnance Survey

raster maps tend to be saved in Index Colour so you may have to change the mode to RGB

Colour. You should find that the image created from your AutoCAD drawing corresponds

exactly to the scale of the underlying image. All you need to do now is position the image

accurately over the map base to create a perfect match.

One final tip worth mentioning at this stage is that if you have mapping information that you

want to combine with an underlying map, consider setting the layer blending mode to "Multiply".

The illustrations above show the effect of the layer blending mode set to "Normal" (the default)

on the left and set to "Multiply" on the right. Notice that



although the colour remains strong, you can see the underlying detail. This gives a much better

effect than simply reducing the opacity of the layer.

To set the blending mode of a layer, click on the layer name in the Layers palette to make that

layer current. Then, click the down arrow at the top left of the Layers palette and choose from

the list of blending modes. As you will see, there are lots of them. If you haven't come across

the layer blending mode options before, try them out and see the various effects you can create.

photoshop matching scales and pixels

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