Tuesday, June 26, 2012

Bushwalking Navigation | How to make a customised, calibrated map from an .ecw map file.

Do you have a Mac computer? Want to know how to use a ecw map file to produce a customised, calibrated, topographic map? Don't know which software to use? Don't want to use a Windows emulator? Not sure which map datum to use?
UPDATE (Tuesday 3 July): advice on calibrating TOPOMaps version 2 ecw files  modified

The following discussion relates exclusively to two highly regarded Mac applications that I, and millions of other Mac users, have been using for what seems like a lifetime. Both are fully supported with user forums, have a prompt response to enquiries, are low cost for what you are getting, and are actively being updated by developers who have a love for Macs. These applications are
It is of course possible to use OziExplorer on a Windows computer or emulate a Windows machine on your Mac, but these are a poor substitute for powerful, user friendly software, running natively on a Mac.

I assume that you are using the latest version of Graphic Converter (GC) (there is a trial period so you can fully test it before purchasing) and have it running in 32 bit mode. You can do this by control clicking on the GC file icon, selecting the Get Info box and then checking the  "Open in 32 bit mode" check box if this choice is available [ see diagram] NB  ecw files won't open in 64 bit mode and you will get an error message if you try to do so.

MacGPS Pro doesn't have a demo version, but it does have a 30 day money-back guarantee.

iOS Applications

MacGPS Pro also has an iPhone/iPad/iTouch version iHikeGPS which is  suitable for New Zealanders and North Americans, due to the availability of free maps. Australians miss out as our maps have to be purchased.

Bit Map (Australian design) allows uploading and viewing of custom maps made using the technique outlined below, using an iPad or iPhone, which is something that few other apps can do. While its plotting features are not as complete as MacGPS Pro, it does allow the plotting of waypoints and routes,  or uploading from an OS X mapping application such as MacGPS Pro via iTunes. Like MacGPS Pro it allows live viewing of your current position using the built in GPS.
Disclaimer: I have no relationship with the  developers of the software discussed above.
For my review of Bit Map click this link.

IMPORTANT: If you have a small (a few map tiles only) .ecw file and the accompanying .map geo-referencing file is in the same  folder, then MacGPS Pro will open and calibrate the ecw file, as soon as your drop it on the MacGPS icon, without any further action on your part. If you don't have the corresponding .map file then you will need to calibrate the newly imported ecw file as in Step 7.
    1.    Drag  the  ecw file from your CD to the GC icon, and dialogue box below will pop up. Select a slightly bigger area than you want by dragging the square corners of the large box [see diagram below] and then position the box by dragging from the middle of the box. Select Downsample."None", then click OK. HINT: It doesn't need to be accurate at this stage. Just make sure the area of interest is inside the selected area.

  2.    Select the part of the image you want by using the selection tool (the dashed square, second line on the right) [see diagram] from the toolbox (bring to forefront by clicking command-K)  and save as (file/save as) a pict file, which is the native MacGPS Pro format. Make sure you have checked the radio button Save Selection Only.[see diagram below]  [Steps: name file, select file format ie PICT, check save selection only]

 3.    Open Mac GPS Pro, set the units (file/unit choices) to the appropriate datum,
which should correspond to the map datum of the paper maps you have or will be purchasing.

If you intend printing labelled maps directly from MacGPSPro as pdfs, then I would suggest you set to GDA94, as this is most likely to be compatible with the maps of others in your group.

 eg GDA94, UTM, kilometres, metres magnetic, click OK. [see diagram below]

    4.    Drag the cropped pict  file from your Finder onto the MacGPS Pro icon.
    5.    A dialogue box will open and ask you to "Set the map's Datum  and Projection type"  [see diagram below] Select GDA94 for most discs recent mapping discs and Transverse Mercator. Click OK

    6.    It will then pop up a "Standard Coordinates for the Map" dialogue box. Enter the grid zone (see image)

and check the 'Store calibration in PICT file" box, then OK [see diagram below]

    7.    You will then have a "Click known Points to Calibrate Map " dialogue box pop up. Check the map datum is set to the same as your hard copy maps before proceeding. [see diagram below]

Complete this for any four easily recognized features (eg trig point, windmill)  located near the four corners of the map, using a hard copy of the map to provide the 7 digit eastings [eg 0262600] and northings [eg 6553800]. Click DONE.

HINT: Make sure you have zoomed in as far as you can, without the map becoming too pixelated, using the zoom icon top right, to increase your accuracy.
VERY IMPORTANT: If you are using an ecw file from a TOPOMaps version 2 disc (South Australian), then be aware that the grid lines on the ecw file are those of the AGD84 datum, as the file has been scanned from pre-94 maps. If however, you open these maps in OziExplorer you will find that the associated .map file (found on the disc) has calibrated the maps as GDA94 hence the disc markings.

The Instructions file on the disc ( who reads the instructions?!) say
The map image displayed (in OziExplorer) has been recalibrated to the Geocentric Datum of Australia 1994 (GDA94). The Easting and Northing readouts for any point at the cursor position will in GDA (94) coordinates.
Please Note:
The map images were scanned from AMG 84 based maps. Therefore, if the cursor is placed at the intersection of two grid lines, the coordinate readout will not correspond to the values of those grid lines. Future map editions will display a grid pattern based on the GDA94 datum and therefore the readout will then correspond to the grid values. See also p25
The warning above is very important, if you normally use grid line intersections as your calibration points, as this will result in inaccurate calibration, with the whole map displaced 100-200m. If you use recognized topographic features this problem doesn't exist.

HINT: To speed up the process in case you need to start over again, keep a record by printing a copy of the map and annotating it with the four corner grid references which you previously entered. If MacGPS Pro won't let you click the DONE button it is probably because you made a mistake entering the GR and will need to start again, or perhaps you have failed to select a point on the map, to register the grid reference.

The Ultimate Goal

Remember that the goal of calibrating a map, plotting waypoints and joining them to form a route using MacGPS Pro is so that you can transfer them to your GPS and use your GPS to find your location relative to these waypoints, on either a purchased map or one printed using the software. For this reason, the map datum set in your GPS MUST match the paper map you are carrying.

My next few posts will explain how to export your data from MacGPS Pro to your GPS, overlay the calibrated map you have produced on Google Earth to help you visualize the terrain and get the UTM coordinates from Google Earth by overlaying with UTM grid.

Visit some of my other related navigation posts

  Creative Commons License This article by Bush Walker is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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