What is Maptime?
- A time for learning about maps
- Emphasis on open source, programming, and web-mapping
The Basics of Geodata, Geodesy, and Geocoding within the context of Open Street Map
If following along, use left and right arrows to navigate.
PLEASE interrupt if you have questions!
OSM is an editable database with global coverage
is licensed under the Open Database License: ODbL
Anyone can edit OSM
So, let's sign up now!
While everyone is doing that...here are some resources for later:
Let's get started!
Two ways to view/edit OSM:
- web (basic): iD
- desktop (advanced): JOSM
Go to OSM
and, assuming you've signed in, click edit
in top left corner
Are you able to scroll around and zoom in (i.e., is our wifi fast enough here)?
If so, find the Carnegie Museum
What are we looking at here?
Geographic data, or Geodata, represents things that have a location.
This can include defined physical or abstract features
- census boundaries
It can also include temporal or ephemeral events
- 911 calls
- cloud cover
- trending, geo-located tweets
There are numerous types of geodata.
Let's talk about the data types you can see right in front of you, in OSM.
Geodata typically is stored and managed in one of two formats:
Raster data stores its geographic information in pixels
Pixels can represent color, height, slope, direction...and many other classifications or gradients.
data is commonly encountered in:
- satellite imagery
- weather data
- digital elevation model
Rasters can be derived from other rasters.
e.g., a digital elevation model can be analyzed to create...
is very powerful
. The format is conducive to complex spatial analyses and modeling:
- kernel density (a.k.a."heat-mapping")
- overlay (e.g., site suitability)
- flow (e.g., surficial hydrology)
In OSM, raster data is provided in the form of satellite imagery.
Try changing the sources and transparencies.
Layers provide sources for validation
Vector data stores geometry, attribute, and location information
No matter how much you 'zoom in', you won't see pixels.
Vector data is dynamically rendered.
Zoom out to see all of Allegheny County
Vector comes (primarily) in three geometric flavors:
- point = point
- line = line or "way"
- polygon = "area"
Editing tools are at the top!
The editing tooltips suggest certain kinds of things are best represented by certain vector types.
Select the Map Data
button and then Map Features
Turn some layers
on and off!
Select a feature on the map.
Look at its attributes
Attributes are an important component of vector data.
Each individual vector feature is like a row in a table. Each table column is an attribute.
|unique feature id
||Carnegie Museum of Natural History
A vector dataset usually stores lots of individual features.
You'll notice that if you have selected a building or street, you'll see on the map the feature is shown as series of of connected points and lines.
Geometry and geographic referencing!
Remember our Market Square buildings?
These numbers tell us where the vectors are located
on the globe...more or less!
This brings us to Part 2:
Geodesy(or, the part where we talk about coordinate systems and projections)
Where do these numbers come from? How do we place geodata on the globe?
WHAT DOES IT MEAN
Let's back up.
Locations are identified using latitude / longitude (lat/lon) decimal degrees.
||horizontal lines, different circumferences
||-90 to +90 decimal degrees
||vertical lines, same circumferences
||0 to 180 decimal degrees
You've probably seen depictions of the globe as a map, both with a lat/lon grid.
So...lumpy, sphere-like 3-D object becomes smooth, planar, 2-D object...?
How do we go from this
Map Projections mathematical equations
for stretching and warping the lat/long grid
to fit the globe to a planar map.
Two Point Equidistant
Suffice to say, there is no way to project the globe onto a map without distorting it.
Distortion is typically worst at the edges of the map, affecting relative scale.
OSM and other web maps use a common projection:
This distortion at the map edge is why map projections are typically specific to a certain geographic extent.
Pennsylvania State Plane (South)
Looks (and functions) well here!
These buildings use EPSG 2272.
But the coordinates shown aren't decimal degrees!
A map projection often also has a built-in conversion to a local unit of distance.
Decimal degrees are not useful for communicating: distances between things, the width of a building facade, or calculating the area of a parcel, etc.
Data in the local PA State Plane projection is stored using feet
WHAT DOES IT ALL MEAN
If you have some geodata
from a local source (say the Allegheny County GIS data available on PASDA
) that you want to put on a web map
, or use to contribute to OSM
Re-project the data first. (to Web Mercator or whichever system the web map is using).
Use desktop software like QGIS
- it's a simple "save as"!
Unless of course, you are starting without geodata.
Which you may very well not have yet.
This brings us to Part 3:
So, maybe you don't have any geodata yet. But you have some addresses, or maybe some street intersections.
Geocoding is the process of associating an address, which is only a bit of text, with geodata that references an associated address(es).
Geocoding can be done using a number of different tools.
The APIs and QGIS plugins allow you to geocode a lot of data at once.
For geocoding to work quickly and accurately, the reference data must have accurate geometry
, an accurate location
, and accurate address attributes
Addressing around Pittsburgh
and in Allegheny County
is fairly convoluted
, for a lot of reasons.
If you want to your data to be gecoded (or reverse-geocoded) properly, you must choose a reliable data source.
The benefit of using an OSM-based geocoding tool is that if you find a discrepancy in geocoding results, it is within your power to fix the base data!
Not as easy with Google.
What else can I do with OSM? Suggestions:
Trace some buildings
Add some POIs (points of interest)
some data for use in desktop software
What not to do:
When contributing, don't copy from Google!
I've got some data, can I load it into OSM?
What did we learn today?
OSM is an open-source, global, and editable geodata source that anyone can use and improve.
Geodata are the elements that make up a map.
Raster data (pixels) and Vector data (points, lines, polygons) are the basic geodata types.
The world is a lumpy 3-D ball, maps are flat 2-D objects, and map projections take care of the hard work of converting from one to the other.
Web Mercator (EPSG 3857) is the standard map projection for data used in a web map (lat/lon)
Local data is often published in the PA State Plane South (EPSG 2272) projection (feet).
Geocoding is the process of converting addresses to geodata.
There many tools out there for Geocoding.
OSM-based tools allow you some control over the quality of the geocoding.
Christian Gass | @GassChristianB
Matt Mercurio | @Geologistatlarg
With special guests Patrick Hammons | @hamhandedly of Code for America and formerly of Maptime Philadelphia
and Bob Gradeck of Open Pittsburgh
Thanks to Maptime HQ
for providing some great basic presentation material (from which much of this has been drawn)
Presentation made with big