Label placement Rules, techniques



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Label placement

  • Rules, techniques


Labels on a map

  • Text, name of map features

  • No fixed geographical position

  • Labels of point features (0-dim), line features (1-dim), areal features (2-dim)



Labels of point features





Labels of point features



Labels of line features

  • Rivers

  • Heights on contour lines

  • Streets



Labels of areal features

  • Countries, oceans, forests, lakes



Three kinds of labels



Label placement

  • Where should the text be placed? Which shape?

  • 20% - 50% of the map production time; can be automated

  • Which rules do cartographers use? - No overlap  readability - Clear for which feature  association - Aesthetical guidelines



Rules according to Imhof (1962)



Problematic cases



Another problematic case



Specific rules point labels

  • Label must be close to point, preferably to the right and above

  • No overlap with other labels, except perhaps in the spacing of the areal label

  • Label may intersect line feature; then line must be interrupted

  • Points close to large body of water must have their label in the water. In other cases there may be no line between a point and its label

  • If it cannot be avoided, a name may be split over two lines of text



Specific rules line labels

  • Label must follow shape of river

  • Label should not bend upwards and downwards consecutively

  • At long line features the label must be repeated

  • No extra spacing between characters, spacing between words of the label is allowed

  • For vertical line features: upward reading direction left on the map and downward right on the map

  • Contour lines: labels must interrupt contour line; top label points to higher regions



Specific rules areal labels

  • Horizontal labeling is good unless this conflicts with the dominant shape of the region

  • Monotonous curving; no inflection points

  • Label must be spread over whole region

  • Non-horizontal labels must be curved

  • Adjacent regions preferably have same shape of label (both horizontal, e.g.)

  • Sometimes an areal label may be outside its region, but not in another labeled region



Strategies for automatic label placement

  • Compute for each feature various candidate positions according to the rules

  • Choose for each feature one candidate position, such that the chosen positions don’t have overlap

  • Sometimes a feature cannot be labeled, and sometimes the label of a feature must be repeated



Candidate positions points

  • Point labels



Candidate positions lines

  • Compute strip with character height above the river which follows the shape approximately, but doesn’t have too high curvature (e.g. with opening, closure)

  • Determine suitable segments in the strip as candidates



Candidate positions regions

  • Determine skeleton (medial axis) of region (polygon)

  • Determine longest path on skeleton as dominant shape

  • Choose middle with suitable character spacing

  • Vary a little for more candidates



Candidate positions regions, horizontal

  • Choose spacing and determine length : height of label

  • Compute largest scaled copy of the label (rectangle) that just fits inside the polygon

  • Put the label in the correct font size in the middle of the scaled copy



Removal part of the candidates

  • Compute those candidate positions that intersect map features significantly, or would give unclear association, and remove them



Reasons for removal



Choosing from the candidates

  • Translate to a graph problem (graph G): - each candidate position of each feature is a node in G - two nodes have an edge in G if their candidate positions intersect - two nodes have an edge in G if their candidate positions are of the same feature



Example



Computing intersecting candidates

  • Each candidate is (approximately) a simple geometric shape

  • Determine all pairs of intersecting candidates using a plane sweep algorithm

  • O((n+k) log n) time, with k the number of intersecting pairs

  • Typically: k = O(n)



Graph problem: maximum independent set

  • Also called maximum stable set

  • NP-hard problem: solving optimally is very time consuming

  • Cannot be approximated with performance guarantees

  • Many heuristics



Heuristics for maximum independent set

  • Select each candidate that is adjacent to a clique, and remove the candidate and clique from the graph

  • Repeat as long as possible (optimal selections)

  • Select candidate that intersects few other candidates

  • Select candidate if a feature has only one (or a few) candidates left

  • Eliminate candidates that intersect many candidates



Refinements

  • Features and labels have different importances  translate problem to maximum weight independent set

  • First place areal labels, then determine candidate point labels and select, then determine line labels and place them (order of decreasing freedom of placement)



Labels in GIS

  • Can label positions be pre-computed and stored, for better efficiency? No, because

    • Which themes are shown is not known beforehand;
    • Which scale and size of the map is needed is not known beforehand


Labels on interactive maps

  • Pop-up labels, or labels only after click

  • During zooming in, remove labels when their feature goes outside the frame

  • During zooming in, allow more and more labels; during zooming out, remove labels of less important features Size of labels does not change!



Commercial packages

  • Maplex for ESRI’s ArcInfo and ArcGIS

  • EverName for MapInfo

  • MapText for ArcInfo, ArcGIS, MapInfo, GeoMedia, …



User Interface of EverName





Output of EverName



Output of MapText






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