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Historical Land Cover of the United States

My first map versions used population density and railroads as a proxy for the state and direction of migration. But what about the destination? What was the land cover like at the time of emigration, given the fact that the large majority of emigrating Bührers hauled from the rural countryside and were farmers?

Land cover of the Northeastern United States around 1870
Land cover of the Northeastern United States around 1870

The map above shows the structure of the landcover around 1870. Red indicates urbanised centres, the yellowish green grassland, brown cropland and dark green forestland. Northwestern Ohio was still largely covered by forests (approx. 60% density), with yet only a recognisable patch of cropland around Bryan. Nowadays the forest is – with the exception of small remnants – gone and replaced by cropland. Agriculture completely changed the surface of Ohio.

The sources for this historical land cover were:

Astonishingly I wasn’t able to find period Ohio maps that include land cover information (e.g. forests).
ISLSCP II Historical Land Cover and Land Use, 1700-1990 provides a source for full land cover (, albeit only at a very low one-degree resolution that wasn’t useful to produce maps. More interesting is Historical Land-Cover Change and Land-Use Conversions Global Dataset (, with half-degree resolution. There is also data for the Original Natural Vegetation of Ohio (, see theme id 3135), essentially almost exclusively forestland of different kinds.

Bührers’ professions over time

What kind of work did the Bührers live off? A total of 815 Bührers in the dataset (10% of which are women) have profession information ranging from general titles such as “farmer” to  very specific ones like “advanced planning manager at funeral home”.

Distribution of professions among Bührers

Professions have been coded with the Historical International Standard Classification of Occupations (HISCO) Tree of Occupational Groups, with modified category names to convey the predominant characteristic of the groups (categories 0-1, 7-9). The number of persons per time slice ranges from 23 (born prior 1750), 63 (born after 1750) up to 262 (born after 1850).

What comes as a surprise is the relatively low share of farmers (category 6) that never exceeded 20%. For most of the time there is a predominant share of production jobs, especially in the 18th and 19th century. Their nature, however, changed over time: e.g. from shoemakers to mechanics (category 8) or carpenters to factory workers (category 9). Not surprisingly some professions have ceased to exist, e.g. “Schalenmacher” (wooden bowl maker).
Women prior the 20th century had expected “woman professions” such as seamstress or nurse, with the odd teacher towards the end of the 19th century. It is only in the 20th century when “Professional, technical and related workers” (categories 0 & 1) become predominant occupations.

There is a clear bias to record/know professions with higher status, e.g. the “Vogt” (the equivalent of a bailiff) that features 6 times for the time slice up to 1750; the same holds true for other professions in the category 2 Administration/Mgmt such as “Gemeindepräsident” (mayor).

What does “family” mean or how related emigrated Bührers are to me

Back in 2011 when I started working with the Bührer dataset it was clear to me that all the emigrated Bührers are “family”, i.e. are more or less closely related to me. A first glimpse at the map’s family tree in 2015 showed that this assumption is somewhat shaky – most of the emigrated Bührers are rather distant relatives.

Degrees of relationship relative to me

A renewed look (see above) gives a more precise picture: relative to me (generation and degree of relationship = 0) the closest emigrated relative is Michael Bührer (generation = 5, degree of relationship = 6) who directly descends from the 10th generation of my direct line of ancestors. The degree of relationship of US persons (living or dead) ranges from 6th to 11th degree. Not that related after all…

Note that indicated degree of relationship according to my calculation (I didn’t find an authoritative source on how to calculate it) doesn’t increase in direct descendancy unless the generation is below 0.

How have Bührers spread throughout Switzerland?

Today, Bührers can be found in various cantons of Switzerland, however, most Bührers still hail from the Canton of Schaffhausen. How did this come about? A time series shows in 50-year steps from 1500 to 2000 (birth dates) where in Switzerland Bührers have been and how they’ve moved.

The earliest recorded Bührer in the dataset is Adam Bührer, who was born around 1533 in Bibern. Long-range migration in earnest starts only in the first of the 19th century.

Note that places have been aggregated at 2018 municipality level (e.g. Bibern and Hofen merged in 2009 with Thayngen). The circle size denotes the number of Bührers.

Revisited: Emigration of the Bührers from Switzerland to the United States

The original dataset from 2011 featured 4557 relevant persons (Bührers, spouses and their children/grandchildren). Up to 2017 the dataset had grown up to 9223 relevant persons, i.e. neatly doubled! This provided ample reasons to redo – with a couple of substantial improvements – the original mapping exercise.

Emigration of Buehrer to the United States – Map 1

The maps shows – as the earlier ones – the Bührers’ emigration to the United States (Map 1) up to the 1880s and subsequent internal migration up to now (Map 2).

Emigration of Buehrer to the United States – Map 2

A legend explains the symbology and the groups of emigrated Bührers with common ancestors. In contrast to the earlier maps there has been no manual tweaking, with exception of the orientation of the immigration arrows. The big challenge is clearly visible in Map 2: the larger number of persons means there’s really too much going on to fit it on one map!

The major changes compared to the earlier version in 2015 are:

  • Emigrated Bührers where matched against emigration/immigration evidence, e.g. passport lists, ships lists at port of embarkation, US immigration records and US census information. This allowed (see 2nd page of legend for emigrated groups of Swiss Bührers) to more accurately determine/narrow down the actual time of emigration.
  • Normalization, completion and geocoding of places was done using a person’s context (e.g. where other relatives had lived/died) to geocode places that otherwise would not be clearly identifiable (e.g. “West Rome Cemetery”). This allowed to geocode substantially more places and hence produces more migrations.
  • Determination of a common male ancestor was combined with deriving the family status for emigrants in a more robust procedure that can accurately handle also more complex cases such as three generations of emigrated Bührers with intermarriage with other emigrated Bührers. As a result some of the groups of emigrated Bührers have changed.
  • Assignment of persons to a time period (generations prior/beyond 1880) was reworked. This allows a flexible ad-hoc definition of periods based on the common minimal birth year of a group of siblings and their spouses. Furthermore extrapolation to ancestors/descendants via a generation offset (25 years) allows the period estimation of persons with no known birth year.

In terms of map-making the are few notable optimizations:

  • The number of distinct Bührer persons per county and common ancestry is indicated with pie charts, allowing to visualize both the total number of Bührers and the repartition by ancestry. QGIS 2.18 strictly speaking still doesn’t support this feature (treating diagrams as labels that float on top), but using a workaround with two map layers in the composer (one below with the base map including the pie chart diagrams and remaining places, the other above with the migration arrows and all the labels) did the trick.
  • Descendants are no longer shown (i.e. their immigration, migration as well as number of persons per county) because they are irrelevant/arbitrary in the US emigration context.
  • Meaningful migration labels are now fully computed, even though a few could still profit from manual tweaking. A custom label ranking for migration paths was developed to prevent label cluttering and lets the important labels prevail (e.g. those with first migration evidence). Place labels were optimized to only show first migration evidence (in red) if this occurred in the respective period.

Showing migration flows

The migration flows of the Bührers lend itself to visualisation with a chord plot, done with R‘s circlize package after a post by Guy Abel.

Bührer migration flows

The plot shows migration flows between regions as well as flows within a region that exceed 200 km. “Regions” are essentially place clusters that have been visually identified on the map. It includes all persons born as Bührer (“Named”) from the dataset, where migration can be inferred based on georeferenced events. Note that a particular person can feature in several flows, e.g. first emigrating to northwestern Ohio with a subsequent migration to Kansas.

In contrast to my maps – that only show the United States – you can also see that some Bührers emigrated to Brazil (around Curitiba) as well as India (Mangalore). Either migration is likely to predate the known emigration to the United States.

Analyzing the Bührer dataset

What data of the available Bührer dataset actually made it on one of the maps? A mosaic plot, done with the vcd package from the open source statistical software R (, gives a quick overview over the relevant factors.

Mosaic plot of the Bührer dataset
Mosaic plot of the Bührer dataset

The plot essentially shows areas proportional to the number of persons, ordered by the emigration status (left) and map # (top). For a given combination the successive blocks in the color red, black and grey denote Named, Married and Descendants persons respectively (see The methodology – preparing genealogical data for maps for explanations). These three categories make up roughly 4’500 persons of the original dataset, with the remainder not being shown. The small circles denote combinations that didn’t occur in the dataset.

A few observations:

  • Only a small fraction of persons in the dataset actually show up on map 1 and 2. This is comes as no surprise, given the large number of e.g. Swiss-based Bührers, “Assumed US” persons as known descendants of emigrants with no place information or “Undetermined” persons where location information could neither be determined nor inferred.
  • The number of Bührers emigrating for the generation prior 1880 (map 1) is significantly larger than the number of emigrating spouses from Switzerland, reflecting the fact that most married once overseas. A look at the category “Third country emigrated to US” indicates that a substantial part of the Bührers – at least for the first generation – preferred to marry other emigrants.
  • There’s very little Bührer emigration happening for the generations born after 1880 (map 2) – almost all Bührers in that period are America-born.

The plot has featured in a small presentation R User Meetup Mosaic plot Thomas Roth 20160803 (includes the R code) in a Zurich R User Group Meetup.

Plotting the map’s family tree

The – with 13 A3 pages very wide – family tree (Family Tree of Emigrated Buehrers) shows all persons that emigrated to the United States including their ancestors as well as their immediate relatives. Persons are aligned horizontally by generation, with oldest generations on the top. Squares denote males, circles females and triangles marriages. Persons represented by black line symbols are shown on the map whereas those with grey line symbols are not. Otherwise symbology follows the one for the map, i.e. line styling indicates category and colours show common male ancestors.

Family Tree of Emigrated Buehrers - Detail
Detail of the family tree of emigrated Bührers

Data for the family tree was prepared in the project’s PostgreSQL database stripping irrelevant persons and families. The family tree was drawn in yEd ( in “Family Tree” mode and styled via Properties Mapper.

GIS data used in the emigration map project

Data for the emigration map project was – except for the genealogical data – all from public sources. Interestingly enough there is also a relative wealth of sources with relevant historical GIS data.

  • Genealogical data from Swiss Buehrer Web Site as of October 2011 ( For practical reasons (notably less work for georeferencing) all persons not linked to the main family tree were removed as well as substantial irrelevant side lines like the Finney emigration from Ireland.
  • Digital Elevation Model DEM (30” resolution) from U.S. Geological Survey ( provided the base for the fairly easy looking smoothed hillshade layer that proved to be the most difficult to produce.
    The four 30” DEM tiles delimited by longitude/latitude (W140N90, W140N40, W100N90 and W100N40) that cover the United States were merged into a single DEM file which was subsequently reprojected and downsampled, all in QGIS.
    In GRASS a moving average was applied to the DEM which was exported as a GeoTIFF. From there, gdaldem was used to generate a hillshade GeoTIFF that received final blurring in Photoshop. Anything but easy.
  • Physical features (1:10 million scale) ocean, coastline, land, rivers and lakes from Natural Earth (
  • US counties from the US Census Bureau (
  • Historical state boundaries (filtered as of 1870) from the Atlas of Historical County Boundaries Project (
  • Historical US railroads (1870) from the Railroads and the Making of Modern America Project (
  • Historical US census data (1870) from the National Historical Geographic Information System NHGIS of the Minnesota Population Center ( The custom download data included e.g. the number of Swiss-born citizens per county and other interesting data that was not yet used in the project.


Software used for the family tree/GIS mapping project

The MacFamilyTree software from Synium ( was used to import, modify, consolidate and analyse the genealogical data. It was also used for the normalization, completion and geocoding of places. Except for MacFamilyTree all other mentioned software are open source.

Data was exported from MacFamilyTree’s underlying SQLLite database as SQL import script with the help of the SQLite Database Browser ( and subsequently imported into a PostgreSQL database ( with a PostGIS extension to add support for geographic objects. Unfortunately there seems to be no high quality GEDCOM-based parser/importer into SQL databases. Data handling and SQL scripts was done using pgAdmin3 (

The very flat data structure from MacFamilyTree was subsequently transformed into a more intuitive data model (“person”, “family”, “place”, “person_event” etc.) that served as a base for the extensive coded analysis and transformation logic in PostgreSQL’s procedural language PL/pgSQL.

All logic (and some data patching) were applied in roughly 40 sequential scripts per object. This repeatable processing proved to be a key success factor given the large number of methodological, coding and data errors encountered in the process that forced reprocessing.

Screenshot of the QGIS project for the emigration map
Screenshot of the QGIS project for the emigration map

All mapping and layout was done in QGIS (, with key features for the project becoming available only in QGIS 2.2. Data came from either PostGIS layers in PostgreSQL or shapefiles from various sources. The original approach to create a raw map that would receive its finish in a vector-based editor was dumped in favour of end-to-end map production in QGIS. This reflects on one hand the growing maturity of QGIS on one side, but also the difficulties to process the incredible amount of paths in its vector-based output in other programs.