Street Atlas (or Battleship) grids in QGIS 2.18

ICES rectangle map

Another guest blog from Liam Mason, based on his lightning talk at the 8th Scottish QGIS user group. You can follow Liam on Twitter via @marinemaps


D5. Miss.

A2. Hit. You’ve sunk my battleship! 🙁

If you’ve used a street atlas or played the boardgame Battleship, you’ll be familar with grid systems using letters for the horizontal (x) coordinate and numbers for vertical (y) coordinate.

Whilst these grids don’t have the resolution of a coordinate system like latitude/longitude or eastings/northings, they allow readers to quickly identify where a street is located on an atlas, or a ship on a boardgame.

Battleship boardgame
Battleship is now geohipster, embracing the hexgrid

Creating these type of grids in QGIS takes advantage of the grid options in composer, the @gridnumber attribute, and a wee bit of maths.

Setting up the print composer

Create your map. In this example, I’ve used data from Natural Earth and projected on World Mercator / ESPG:3395 (because I want meridians and parallels of latitude & longitude to appear straight)

Example map in QGIS

Create a new print composer using Project > New Print Composer (or Ctrl & P)

QGIS project menu showing New Composer windo

Use the Add new map button in Composer to add the map content.

Add New Map button

(Leave space around the map for the graticules.)

Map on print composer showing white space

Adding a latitude / longitude graticule with custom format

Click on the map (boxes will appear in corners to show it’s selected). Click the Item Properties tab, scroll down to Grids, and expand using the wee triangle.

Add a grid using the + button. Double-click on the name (Grid 1) and rename to something like ‘Grid lines’

Item Properties showing Grid

I want my grid to use decimal degrees on WGS84 datum, so I select a CRS of EPSG:4326.

Using an X interval of 1 decimal degree and Y interval of 0.5 decimal degrees produces a grid that’s roughly rectangular.

Grid lines

Scroll down the item properties (past grid frame), and tick Draw coordinates. I only want the graticules down the right hand side and bottom, so I set Left and Top to Disabled.

Draw Coordinates

I want a more truncated latitude & longitude and use a custom format.

Set Format to Custom, use the expression button (to right of format) and expression based annotation .

expression annotation lat-lon

abs(@grid_number)
|| '°'
|| CASE
WHEN @grid_axis = 'x'
THEN
IF (@grid_number > 0, 'E' , 'W')
ELSE
IF (@grid_number > 0, 'N' , 'S')
END

This expression returns an absolute value of @grid_number. Since the CRS is EPSG:4326, @grid_number is decimal degrees, and would normally return a negative value for longitudes west of 0.

The rest of the expression concatenates (||) the degree symbol, and a conditional statement.

When the grid axis is X, and value is greater than zero, append E for east, otherwise append W for west. If grid axis isn’t X (i.e Y) and and value is greater than zero, append N for north, otherwise append S for south.

Now I have latitude and longitude down right and bottom sides in the desired format.

Lat Lon Grid

Adding a numeric value

Scroll back up and add an other grid using the + button. Rename this one to something like ‘numeric’. This grid will be used for the numeric grid reference.

This time I only want annotations, so I change the Grid type to Frame and annotations only. The CRS again will be EPSG:4326 and the Y interval will be 0.5 but I want the numeric annotation to be in the middle of the grid lines, so add an Offset of 0.25 (half of interval).

numeric grid settings

Scroll down the item properties to Draw coordinates. I only want the numeric graticules down the left so I set Right, Bottom, and Top to Disabled.

numeric grid coordinates

Set Format to Custom, use the expression button and use expression based annotation . Note in the example above, 49°N is most southern latitude.

Expression annotation for numeric grid

((@grid_number-49)+0.25) / 0.5

The expression calls the grid number (decimal degrees including 0.25 offset), subtracts the lowest X value (49), adds the offset (0.25), and divides by the coordinate interval (0.5). (For values to start at zero, subtract the offset)

This creates a numeric grid which increases in a northerly direction. For a grid to increase in a southerly direction, use:

abs((@grid_number-62.5)+0.25)/0.5

This similar to above, but substracts the largest X value and converts the number to an absolute value)

Adding a letter value

Scroll back up and add an other grid using the + button. Rename to something like ‘letters’.

Again I only want annotations, so similar to previous settings except the X interval will be 1 with an Offset of 0.5

Letter grid

Scroll down the item properties to Draw coordinates. I want the letters on the top so I set Left, Right, and Bottom to Disabled.

letter grid coordinate settings

Set Format to Custom and use the expression button to select expression based annotation . Note in this example, 13°W was the furthest west (-13)

expression for annotation letters

substr('ABCDEFGHIJKLMNOPQRST', ((@grid_number- -13) + 0.5) / 1 , 1)

The expression works like the numeric grid in that it calls the grid number (decimal degrees including offset), substracts the smallest Y number (-13), adds the offset (0.5), and divides by the coordinate interval (1).

This is nested within a substring query which calls a single character (defined by the length of 1) for the corresponding grid number . For example where the grid number is 3, the the third character, C, would be displayed.

And there you have it! A real world version of Battleship, (or a custom street atlas.)

Alphanumeric grid

The ICES statistical rectangle grid (a grid with more than one character)

My incentive for looking into these types of grids was to annotate fisheries maps with ICES statistical rectangles, which use alphanumeric codes to reference areas of the north east Atlantic.

I use the same X & Y internal and offset values as the examples above but different format expressions.

The numeric encoding is familiar enough. The differences from the examples above is that the smallest X is 36N and I’ve used left padding (lpad) to force leading zeroes to be displayed (eg 01, 02, 03…)

lpad(((@grid_number-36)+0.25) / 0.5, 2, 0)

The letter encoding is a wee bit different.

substr('A0A1A2A3B0B1B2B3B4B5B6B7B8B9C0C1C2C3C4C5C6C7C8C9D0D1D2D3D4D5D6D7D8D9E0E1E2E3E4E5E6E7E8E9F0F1F2F3F4F5F6F7F8F9G0G1G2G3G4G5G6G7G8G9H0H1H2H3H4H5H6H7H8H9J0J1J2J3J4J5J6J7J8J9K0K1K2K3K4K5K6K7K8K9L0L1L2L3L4L5L6L7L8L9M0M1M2M3M4M5M6M7M8', (@grid_number - -44) / 0.5 , 2)

The smallest Y is 44, but this time I need the grid value to increase by two at each step. I achieve this by dividing by 0.5 (half the interval of 1), I don’t correct for the offset (otherwise the first value would be 2 instead of 1), and the substring now has a length of 2.

ICES rectangle map

The finished article, ICES rectangle annotations which work anywhere in NE Atlantic.

3D mapping and bathymetry styling with QGIS 2.18

This is a guest blog from Liam Mason, a spatial analyst with Marine Scotland.  Some of his other data visualisations can be seen on his @marinemaps Twitter account or the Marine Scotland Maps portal maps.marine.gov.scot


[Edit: the tutorial was modified to use the GDAL-based DEM (Terrain Analysis) tools instead of Raster Terrain Analysis]

I love mapping bathymetric data in 3D. It’s almost magical, the ability to draw back the veil of the sea and reveal the mysterious landscapes below.

Yet, it’s remarkably simple to do using QGIS 2.8 or higher.

3D bathymetry map of inner Firth of Forth.
Bathymetry of inner Forth made using QGIS 2.8 and qgis2threejs

Continue reading “3D mapping and bathymetry styling with QGIS 2.18”

Creating Cartograms using QGIS 2.18

Cartograms

First a confession, I don’t like cartograms, at least not the kind where complex boundaries are warped sometimes beyond recognition. However they do have their place and where there aren’t the extremes in the data a cartogram can be a great data visualisation. It is outliers or extreme values that cause the maps to be distorted beyond usable recognition, and therefore look bad (see below).

Cartogram PaullHennig2016WorldMap.OAha.CC-BY-4.0
By John Paull and Benjamin Hennig (http://orgprints.org/30187/) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Anyway, if your data isn’t going to produce a map like this then you can use the Cartogram plugin in QGIS. This allows you to quickly turn your map into one that removes the misrepresentation of small areas hiding large values. Continue reading “Creating Cartograms using QGIS 2.18”

Creating a statistical dot density map with QGIS

Dot Density maps:

These maps display the density and distribution of a phenomena over a geographic area. The markers, usually a dot or cross, represent the occurrence or an aggregation of occurrences which are then randomly distributed across distinct regions of the map. Colours can be used to represent different classifications to add an extra dimension to the map.

The example above shows votes to remain (red) and leave (blue) the European Union in the London Boroughs. Each dot represents 100 votes and these have been randomly scattered within each borough to represent the density of votes. Continue reading “Creating a statistical dot density map with QGIS”

Creating 3D choropleth or prism map in QGIS 2.18

Contains OS data © Crown copyright and database right (2017)

It is possible to very quickly render any choropleth map you make in QGIS in 3D using the Qgis2threejs plugin. Essentially the plugin allows you to turn a numerical attribute as a “height” for your data. The results from the plugin are outputted as an html page which can be easily shared in a folder or placed on the web for people to view. Continue reading “Creating 3D choropleth or prism map in QGIS 2.18”

Creating a Proportional Symbol map in QGIS 2.18

Creating proportional symbol maps in QGIS is made very easy in QGIS with two main methods for making them. You can use basic Single Symbol style with the  Size Assistant in the Data Defined Override,  or you can use the Graduated style and choose Size as the method of gradation. Below is a set of instructions on how to use either method to create proportional point symbols for your map. Continue reading “Creating a Proportional Symbol map in QGIS 2.18”