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How to geo-reference a map

If you import a screenshot or scanned image of a map into PTMapper then it will not be geo-referenced, so you will not be able to use the GPS on your device when surveying. You will sometimes find the same problem with DXF files (from CAD software) that your client supplies to you.

PTMapper has an option to geo-reference maps like these and this process will work for screenshots, scanned maps and DXF files.

Note: if your map is based on an image (e.g. a  scan or screenshot) then your image should preferably be saved as either a JPG or a BMP file.

If you are using a PDF then you will need to convert it to a JPG or a BMP file first.  PTMapper will not accept PDF’s.


To be able to geo-reference your map, you will need to find out two bits of information about the area that your map covers:

1) The grid reference numbers of a recognisable feature on your map  e.g. the corner of a building (this is used to geo-reference your map).

2) A known measurement between two recognisable points on your map  e.g. the distance between two buildings (this is used to scale your map).

A long measurement is preferable – the distance between two buildings that are 100m apart would be far better than the length of a 5m garage.


The website gridreferencefinder.com is an ideal place to find out both of these bits of information.

The following instructions cover how to use this website to obtain the details needed to geo-reference a map:

NOTE: To be able to use this website you will need either the postcode or the street name and the city/town/village for the area that your map covers.

 

1. Finding a grid reference (refer to the next screenshot for the steps below)

1) On the website you will see options on the left of the screen to type either a post code or an address – do this and then click on “Go”.

2) Once the area is displayed in the main map window, look for recognisable features that match your map.

3) Once you recognise the area that your map covers, zoom to the maximum level and right click on a suitable feature (in the screenshot shown below I have chosen the corner of a building). This will display an information box like the one shown below – note the grid reference for the feature that you clicked on.

find-grif-ref

 

2. Measuring a distance between features (refer to the next screenshot for the steps below)

Now look for a second recognisable feature that can also be found on your map (e.g. another building) – you may need to zoom out to do this.

NOTE:  You will need to be able to see both this feature and the first feature that you clicked on, so don’t zoom out too far.

 

1) Click on the “Measure” button.

2) Select “Metric” and then move this box to one side so that you can see the main map window.

3) Click on the first feature and then click on the second feature – this will draw a line from one to the other.

4) Click on the “Stop Drawing” button in the box that you moved to one side.

5) In the box it will now show the length of the line that you drew – note this figure down.

 

You now have the two bits of information that you need to geo-reference your map in PTMapper – a correct grid reference and a known distance.

 

find-grif-ref

 

3. Geo-referencing your map in PTMapper

1) Begin with a blank map using FILE>NEW MAP.

  1. a) If you are planning to use a DXF then use FILE>IMPORT>IMPORT DXF FILE… to import the DXF into your blank map.
  2. b) If you are using an image then create a new layer on your blank map and call this new layer “Raster”.

Then import the image using FILE>IMPORT>IMPORT RASTER FILE…

You will see a warning message that the image is not geo-referenced – ignore this and click “OK” to continue.

NOTE: before continuing – move the Pencil to the “Pen Parking” layer and then lock ALL of the other map layers (don’t worry about the “Page Outline” layer).

 

2) Click on TOOLS and then TRANSFORM (refer to the following three screenshots for the steps below).

3) Click on the same feature that you did in gridreferencefinder.com and then in the “New Easting” and “New Northing” boxes type the grid reference figures that you got from the website (this will give the map its true geo-referencing).

4) Use the Draw Line tool to draw a line between the same two features that you drew the line between on the website. In the “Line Length” box type the distance in metres that you noted earlier (this will give the map its true scale).

5) Click on the “Transform Map…” button.

5) Click on the “New Grid Label” dropdown arrow, scroll down the list and select “OSGB”.

6) Click on the “Change Grid Label” button and then click on “OK”.

 

grif-ref

line-measurement

grid-system

 

4. Final Checks

To test your map:

  1. a) For DXF’s – unlock a suitable layer (e.g. buildings) and select an object on the map, then click on the Google Earth link to check that it fits the correct location and size.
  2. b) For maps based on images – use the Draw Line or Draw Polygon tool to draw around a recognisable feature (e.g. a building) and use the Google Earth link to check that it fits the correct location and size.

If your map is correct then save it.

 

Don’t forget to delete the line that you drew earlier..!

 

 

 

Updated on 01/08/2017

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