Information and Advsiory Note Number 62, November 1996
1.1 This Note sets out to describe those aspects of Phase 1 survey which relate to the capture and storage of habitat information in digital format.
1.2 The development of the Phase 1 survey method was initiated in the 1970s by the Nature Conservancy Council for mapping terrestrial habitats. The first manual on habitat mapping was produced in 1986. The Phase 1 methodology and classification scheme referred to in this Advice Note is taken from ‘Handbook for Phase 1 Habitat Survey - A Technique for Environmental Audit’ NCC 1990.
1.3 The traditional outputs from Phase 1 habitat survey are hand-coloured paper maps (usually at 1:10,000 scale), a survey report and target notes related to the map by a reference number and national grid location.
Habitats identified during the survey are described on a ‘Map Sheet Record’ form. This includes details about the base map and the extent of each habitat or feature surveyed. This document forms part of the written report.
The combination of paper based mapping, the presentation of areal and linear statistics and target notes related to the survey map make Phase 1 habitat survey suited for inclusion within a geographical information system (GIS). The interpretative report which may accompany the survey, could be referenced but would not be stored on GIS.
2.1 The Phase 1 habitat classification is hierarchical. Based principally on vegetation, it is augmented by reference to topographic and substrate features. Habitats can be subdivided by species coding.
2.2 A total of 115 areal, linear and point features are recognised by Phase 1. These are classified into 10 broad groups.
2.3 According to the Phase 1 classification each habitat or feature is identified by an alphanumeric code. A lettered code is also provided by the Phase 1 classification but, unlike the alphanumeric code, it may not be unique to each feature and so is of less value in a GIS environment.
2.4 The Phase 1 classification recognises a small number of mosaics. However, there are situations where mosaics occur on a Phase 1 survey which are not recorded as a class in their own right: e.g. where bracken occurs as a scattered feature within woodland. In such a case, features which are primary classes in their own right are regarded as occupying the same location on the ground. The distinction between mosaics and primary habitat classes is important to the GIS and is discussed in more detail in section 4.
2.5 Areal, linear and point features can be represented through Phase 1 mapping. Some features, such as scattered scrub, may be represented as areal, linear or point features. The interpretation of the feature will depend upon the context in which it is shown.
3.1 At the digitising stage the GIS will capture and store habitat information as either a point, a line or a polygon (area). Each of these spatial feature types will be stored as a separate layer or theme. Each unique spatial feature will be attributed with the Phase 1 alphanumeric code for the habitat feature it represents.
3.2 Digitising should not attempt to reproduce the hatching or symbols used to describe habitat features. Only the position of points, linear extents or boundaries should be captured.
3.3 Phase 1 surveys are normally mapped at 1:10,000 or less frequently 1:25,000 scale. It is a relatively simple task to register the paper map for digitising by using the full Ordnance Survey national grid reference shown at the corners of each map sheet.
3.4 All individual map sheets making up the survey should be edge-matched. Where areal or linear features extend over more then one map sheet, the attribute code should be consistent on each map sheet.
3.5 Annotation (i.e. lettered codes) shown on the Phase 1 survey map should not be recorded. Any text required for map presentation should be stored as an attribute of a habitat feature. This allows map annotation to be viewed at scales other than the source capture scale.
3.6 Map features which are shown on the underlying Ordnance Survey map but which do not represent a boundary or position of a habitat feature should not be digitised.
3.7 A general rule regarding the accuracy of linework is that a plot of the final digital map when, overlain on the paper base map at the same scale, must not visibly deviate from the original.
3.8 For scattered features such as scrub or bracken, the person digitising must decide whether the feature should be represented as an area, line or point.
3.9 Where the mapping of habitat areas produces an enclosed polygon which is not classified (e.g. when a dedicated woodland survey contains an area of moorland which was not mapped by the surveyor) the polygon should be given an alphanumeric code of null. This will help ensure consistency of feature attribution within the GIS.
3.10 A Phase 1 survey may record the extent of built features which are not shown on the base map. Alternatively, built features such as roads which are shown on the base map may be left blank. Both approaches are acceptable for storage within a GIS. Any area of built land on the base map that is also completely enclosed by classified habitats should be recorded as a built area in its own right.
4.1 The alphanumeric code is the main attribute for the identification of habitat features. It should be defined as a character field with a width of 20 in order to provide enough room for mosaic codes.
The following rules should be applied to ordering mosaics which do not have a unique alphanumeric code:
These rules will ensure that, where mosaics are recorded, the primary habitat or the vegetation habitat is recorded before secondary, physical, linear or man-made features.
4.2 Metadata (information describing the survey) should be recorded as a separate file. The metadata file should record the type of information held on the paper ‘Map Sheet Record’ form which forms part the survey report.
5.1 Target notes are represented on a Phase 1 habitat map as a red circle with a dot in the middle. On some maps, target notes have a number next to them for reference. On other maps there is no numeric reference and the target notes are located by a national grid reference on the target note record sheet.
Target notes can provide valuable information on habitat condition, species distribution or physical morphology. As such they should be incorporated into the GIS where available. As point features, target notes can be held in the same layer as the point habitat features. Target notes have a unique alphanumeric code and can be stored in the same manner as any other point feature.
5.2 Relating target notes to the information recorded on a target note record sheet requires a unique reference code for each target note. The reference number should be unique not only to the map sheet on which the target notes appear but to the whole survey area. Ideally the target note reference should be unique nationally, allowing a number of Phase 1 habitat survey datasets to be joined together.
The simplest means of providing a uniquely referenced target note is to name each target note after the map sheet and a sequential number for that sheet. (e.g. target notes on Ordnance Survey 1:10,000 sheet NC62SE would be referenced as NC62SE1, NC62SE2 … etc.) Within the point feature theme, target notes should be coded in a similar manner to mosaics by recording the alphanumeric code and the unique target note code separated by a forward slash: e.g. J5/NC62SE1.
6.1 The handbook for Phase 1 habitat survey (NCC 1990) describes the colours, shading, linework and symbols which should be used to represent habitat information. This is a useful reference and greatly assists the consistency of presentation of the paper maps.
A paper map can only represent a single view of the habitat information. A GIS stores information about a habitat survey and can be instructed to produce many different views of the data. Each view can be tailored precisely to a particular question: e.g. produce a map showing the distribution of improved grassland and arable parcels within the survey area. Or the GIS can produce a view which classifies the information based solely on the main groups (A-J). Since the GIS holds information on areal, linear and point information in separate layers or themes, maps can be produced with or without those elements of the survey.
6.2 If a user of the GIS habitat survey requires a view similar to a traditional map then this can be done. Although, by default, the GIS will not contain a knowledge of the Phase 1 colour and symbol scheme, such a shading scheme can be developed for the GIS. Forcing the GIS to emulate paper and pencil methods of mapping is generally not the most effective means of using the technology.
7.1 Phase 1 survey outputs have traditionally been produced without the express intention of digital storage. While the methodology is well suited to GIS, there are a number of practices which would make GIS storage a more straightforward process.
A Phase 1 survey should be undertaken with the intention of storage on a GIS. It should consider the following:
Surveyors often do not close parcels on the periphery of a habitat survey: e.g. on a river corridor survey, habitats on the valley sides may be of lesser interest and therefore shaded roughly and left unclosed at the outer edges. For GIS use, all areal habitats should be completely enclosed by a bounding line.
In the illustration above, a number of solid shaded and cross-hatched habitat types can be seen. Where solid shading is used the extent of habitats is clear. The hatched area, representing coniferous plantation on the valley sides, does not have a definite boundary. The placement of the boundary will have implications for any area statistics produced from the GIS.
In the second illustration, mosaics are represented on the map: i.e. scattered bracken (crosses) under broad-leaved woodland (hatched). In such instances the surveyor should attempt to represent the scattered class in a manner which will aid digitising staff to identify the area of mosaic and represent it as a discrete area within the GIS. This can be done by increasing the density of the scattered class to allow a discrete parcel of mosaic to be identified.
Where scattered features cannot be identified as a parcel then they should be recorded as point features.
The surveyor should aim to produce a final map with the highest level of clarity possible. The person digitising the map should be left in no doubt as to the location of boundaries and the coding of habitat features.
8.1 An introduction and overview of GIS within SNH is given in Information and Advisory Note No. 34. General guidelines for map presentation for inclusion within GIS are given in Information and Advisory Note No. 33.
NCC 1990. Handbook for Phase 1 Habitat Survey - a technique for environmental audit. England Field Unit, Nature Conservancy Council