PlotSwap: Plot Files Explained @ ExplorOz

PlotSwap: Plot Files Explained

Submitted: Friday, May 16, 2008 at 18:43

This tutorial will provide some technical insight into the workings of an OziExplorer plot file (.plt) and an OziExplorer Map file (.map). You will see a line by line explanation of the data elements and how the three files which are featured in the ExplorOz PlotSwap system hangs together. Hope you enjoy the tutorial.

What is a Plot File (.plt)?

A plot file (.plt) is basically data storage of GPS plot points. You can open an OziExplorer plot file with a text editor such as Notepad and see how it’s constructed. The values in the first five rows represent:

Line 1: File type and version information

Line 2: Geodetic Datum used (e.g. WGS 84)

Line 3: "Altitude is in feet" - just a reminder that the altitude is always stored in feet

Line 4: Reserved for future use

Line 5: Multiple fields for things like width of track, track colour (RGB) and other displaying data.

After row five, you will see the plot point information in a chronological order with each line representing one point. The data values are separated by commas (resembling a column-like structure) and if we look from left to right they are:

Column 1: Latitude - in decimal degrees

Column 2: Longitude - in decimal degrees

Column 3: Code - 0 if normal, 1 if break in track line

Column 4: Altitude in feet (-777 means no altitude was recorded)

Column 5: Date and Time as absolute number - First value is the number of days that have passed since 12/30/1899. The decimal part is a fractional representation of the time of day (e.g. 0=12.00am, .25=6.00am, .5=12.00pm, .75=6.00pm, etc)

Column 6: Date shown as text (Note: Column 5 is only read in by OziExplorer)

Column 7: Time shown as text (Note: Column 5 is only read in by OziExplorer)

Plot Files and PlotSwap

PlotSwap is an interactive area of the ExplorOz website where people who use mapping software can share plot files. As part of this system, we also manipulate the raw plot file data for you, producing dynamic, fully geo-referenced Topo 250K digital maps.

There are hundreds of plot files on our system and these can be downloaded using PointPlace Credits. You can choose and select between downloading the plot file on its own or the plot file with the Topo 250K digital map image file (.png). If you decide to download the map file, you will get three files including the plot file (.plt), the map image file (.png) and the OziExplorer map file (.map) which is a text based file needed for OziExplorer to point and calibrate the map image file (.png).

What is an OziExplorer Map File (.map)?

You can also view the .map file in a text editor like Notepad is see how it’s constructed. Looking at this file line by line reveals:

Line 1: Version of File (e.g. OziExplorer Map Data File Version 2.1)

Line 2: Title of Map (e.g. Brisbane Region)

Line 3: Links to the map image file (e.g. D:\MyMaps\BrisbaneRegion.png)

Line 4: Usually says 1 ,Map Code

Line 5: Geodetic Datum used (e.g. WGS 84)

Line 6 and 7: Reserved data fields

Line 8: Magnetic Variation entry - degrees, minutes

Line 9: Map Projection (technical map and calibration settings)

Line 10 to 39: Calibration points, there are always 30. Only 9 points can be modified from within OziExplorer but OziExplorer will use the full 30 if they exist.

Beyond line 39, there are projection setup parameters and map feature lists.

ExplorOz Plot File and Map Demo (Free Download)

We have provided a free download of three files for you to view in OziExplorer. It is a good idea to keep these files together in a folder which you may want to name ‘Maps’. If you do not have OziExplorer, please visit their website and download the free trial edition. Once you have installed OziExplorer, you must load a map file first, but not the map image file (.png). Instead load the OziExplorer Map file (.map) and OziExplorer will automatically load the map image file for you. You can then proceed to load the plot file (.plt).

Have fun!!!

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