The invertebrates of Prosperous Bay Plain, St Helena a survey by Philip and Myrtle Ashmole September – December 2003 Commissioned by the St Helena Government and financed by the Foreign and Commonwealth Office



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Appendix 1.


Details of sites studied by Philip and Myrtle Ashmole on and near Prosperous Bay Plain, St Helena, September to December 2003
Note. Locations are indicated by GPS reading, with latitude and longitude given in degrees, minutes and decimal fractions thereof.
Central Plain West (PBP1) GPS 15 57.251 S, 05 39.472 W

This site is at the southern edge of the Central Basin of Prosperous Bay Plain (PBP). Altitude c. 310-320 m. This is a hybrid site, including both flat dusty ground with eroding sand dunes and dry runoff channels, and the lower part of the rim of the basin (below main creeper slope). The slope shows similarities with those at PBP2 and WS12, while the dusty plain is comparable to SP8, SBS22 and the nearby BYD24. The flat area has a substrate of fine dust where a cane goes in easily and which provides little cover but could be used for burrows (no lycosids were found, but could have been missed). Patches of small stones increase in frequency up the, which has a gradient up to c.10 degrees and is mainly gravel and sand with larger stones mainly <10 cm across, but very few large rocks; plenty of retreats for arthropods and some seem suitable for Latrodectus, but not found. Wirebird seen.



Vegetation. On dusty flats 10-20% cover. Plant ranking: 1 Suaeda fruticosa; =2 Atriplex spp, Carpobrotus edulis; 4 Hydrodea cryptantha; =5 Chenopodium sp, Mesembryanthemum crystallinum. Vegetation on rocky slope 50-60%, =1 Suaeda, Carpobrotus; 3 Atriplex; 4 grass; =5 Hydrodea, Opuntia sp.

Human influence. Some tyre tracks. Mice trapped and seen; old donkey droppings.
Central Plain East (PBP2) GPS 15 57.129 S, 05 39.172 W

A site in the easternmost part of the Central Basin of PBP and including part of the slope of Stone Hill ridge, where most of the collecting was done. Altitude c. 305-320 m. The lower part is flat and featureless, with a substrate of gravel over dust, but below the surface it is more soil-like than PBP1 so it is quite hard to get a cane in. On the slope this grades to larger stones with emerging bedrock in places.



Vegetation. On slope 30% cover. Plant ranking: =1 Suaeda, Atriplex, Carpobrotus; =4 Portulaca oleracea, Euphorbia heleniana. On flat <10% cover, plants: 1 Suaeda, =2 Atriplex, Hydrodea cryptantha.

Human influence. There are wheel tracks on the flat part but the slope is undisturbed. Mice trapped.
Cliff Top (PBP3) GPS 15 57.318 S, 39.873 W

Part of the cliff edge east of PBP. An exposed and windy site facing east immediately inland from a vertical cliff, with steep, stony, dry slopes and a small level plain (~half acre). Altitude c. 290-310 m. Gradients between horizontal and 30o. Substrate of unstable, fragmented black rocks (varied sizes) over bedrock of solid rock that is exposed in many places. The broken rock is much paler where it is disturbed as a result of old guano seepage. On the few flat areas the surface rock is less dominant and the substrate is of small whitish chips overlaying brown dust. The exposure and dryness results in generally few animals, but there are some sheltered places with better retreats.



Vegetation. 5-10% cover. Plant ranking on rocky parts: =1 Carpobrotus, Atriplex semibaccata. On the small flat the only plant is Hydrodea cryptantha. Also present: Cotula coronopifolia, Portulaca oleracea, a thistle, and frondose and encrusting lichens.

Human influence. Some tyre tracks. Rock collection in the area. Rabbits are present but we don’t know about mice (traps lost).

Comment. Cliff top influence seems to bring in Ptinidae and be good for micromoths. But we didn’t get mogoplistine crickets.
Earwig Gully (PBP4) GPS 15 57.459 S, 05 39.059 W

A gully southeast of the Central Basin, which leads into the main one running SE towards Gill Point. Altitude c. 290-310 m. Aspect SE, with the upper parts of the site rather exposed. Some small cliffs with overhangs, disintegrating into the highly unstable 30o slopes; an erosion gully running down to the main one, and some almost flat and fairly stable dusty areas. The cliffs are of hard black rock, with lots of evidence of guano. Flat areas of dust with white and dark stones of varied size. Plenty of retreats for invertebrates on the slopes; burrows but few other retreats in the dust.



Vegetation. 30-40% on slopes, 10% on flat area. Plant ranking on slope: Atriplex; =2 Suaeda, Carpobrotus; also present grass, varied lichens, occasional Cotula coronopifolia, on Nicotiana glauca nearby. On flat area: 1 Suaeda; 2 Hydrodea. Also present grass: Atriplex, Mesembryanthemum, Carpobrotus.

Human influence. Various tyre tracks and small excavations. Evidence of substantial stone collecting, and several large rocks have been removed. Some areas with deeper dust where rabbits have been busy, but mice not noted. Comment. A rich site including a variety of habitats.
Upper Bone Gully (PBP5) GPS 15 57.272 S, 05 39.771 W

Gully in western rim of PBP, bordered on south by Dry Gut. Altitude c. 340-350 m. The site comprises the upper part of a shallow gully (sloping ESE) on the edge of a plateau at the eastern extremity of Woody Ridge. This gully provides the only substantial inflow of water to the Central Basin from its surroundings and carries much silt and stony debris. It eventually runs over the edge of the plain into Fishers Valley past the concrete hut, but this capture is evidently recent since no substantial ravine has developed; previously the plain may have drained – on rare occasions - southeastwards into the gully running towards Gill Point. In the site there are no cliffs or large rocky areas. Slope <10 o. Flat area at top is stable, but there is active erosion towards and in the gully. Rather exposed, with only shallow sheltered hollows. Substrate is mainly fine dust, but there are patches with scattered stones. The dust is easily worn away in gullies, but digging down it seems like relict soil. Good retreats for burrowing animals, and some large stones. Wirebird seen.



Vegetation. c.90% cover. Plant ranking: 1 Carpobrotus; 2 Suaeda; =3 Atriplex, Chenopodium sp; 4 Mesembryanthemum. Also present: Opuntia, Euphorbia heleniana, Lantana camara.

Human influence. Vehicle tracks nearby. Saw mouse, old donkey droppings, rabbit droppings, canaries.

Comment. A rich and unique site that needs protection as part of the basin ecosystem.
Trig Point (PBP6) GPS 15 56.765 S, 05 39.044 W

North PBP towards north of original Shelco airport footprint, due west of King and Queen rocks. Altitude 300-320 m. This exposed and barren site includes a rock ‘mesa’ likely to be demolished, with steep, fragmented and unstable sides and some overhangs. Between the hills are flat dry plains of mud and grit that hold water after rain; there are small rocks on lower ground round the mesa. The level summit of the mesa has few loose rocks suitable as retreats, and although there are much loose shaley rock on the slopes the instability is an adverse factor.



Vegetation. <5% cover. Plant ranking: =1 Suaeda, Atriplex; 3 grass (2-3 spp); 4 Carpobrotus; 5 Portulaca olerosa; 6 Hydrodea cryptantha.

Human influence. There is a 4 x 4 track through the site and rock collecting nearby, but little disturbance of the site. Trapped 1 mouse; didn’t see rabbit droppings.

Comment. The most inland site that has mogoplistines. It does not have ptinids.
Gill Waterfall (PBP7) GPS 15 57.750 S, 05 38.856 W

Rocky top of cliff and steep (45 degree) slopes down to gully just above lip of waterfall on the way to Gill Point (not quite where the Belgians searched). Altitude 210-250 m. Very exposed to prevailing winds. Bedrock with horizontal stratification and moderate stability; some smaller stones and shallow dust pockets, but no deep soil, so hard to find places for pitfalls. A more uniform site than others. Retreats for animals rather scarce and probably not humid, since surface rocks tend to be convex.



Vegetation. 10-20% cover. Plant ranking: 1 Carpobrotus; 2 Atriplex; 3 Opuntia sp; 4 Cotula; 5 Suaeda; 6

Hydrodea. Also present: encrusting and frondose lichens.



Human influence. Very little disturbance, though a footpath goes nearby. Rabbit droppings and cat scats, but no mice in traps – one not even eaten or sprung.

Comment. Traps include many woodlice, and the first reasonable sized Dysdera and an unknown gnaphosid that later turned up at PBH18. Cliff top had a lot of Nysius and micromoths.

Samphire Plain (SP8) GPS 15 57.154 S, 05 39.425 W

A dusty plain in the middle of the Central Basin of PBP. Altitude 305 m. Flat featureless landscape. Windy but with stable substrate of deep fine dust with small drying-out cracks indicating that surface is sometimes wet. Hardly any rocks, but some retreats for animals are available in the vegetation, and the dust is good for burrows.



Vegetation. 50% cover. Plant ranking: 1 Suaeda; =2 Mesembryanthemum, Chenopodium sp; 4 grass. Also present: Carpobrotus, Opuntia, Lantana and (quite abundantly but seasonally dead) a radiating prostrate plant that is probably another chenopod.

Human influence. Well-worn vehicle track through the site. Donkey droppings.

Comment. This site epitomises the desert Suaeda habitat with an invertebrate fauna dominated by lycosid spiders. Of these, Hogna nefasta is dominant, but “lurkers” Trochosippa sp also present locally, and a also a Brevilabus sp.
Bryan’s Rock (BR9) GPS (est.) 15 55.93 S, 05 39.56 W

Eroded rocky slopes with northeast aspect below the upper cliffs surrounding this part of Horse Point. Altitude 250-300 m. Somewhat exposed, but not to the prevailing wind. Slope varies, but between 5 and 45o. Fairly stable, with loose surface rocks and stones on bedrock. Rocks usually broader than deep, but with some lumps of conglomerate and often mixed with whitish powder that is clearly old guano in some places. Rocks are often in piles and soil blown out from under. Plenty of retreats for animals. White Terns Gygis alba in the area.



Vegetation. 5-10% cover. Plant ranking: 1 Atriplex; 2 Suaeda; 3 Carpobrotus. Also present: Opuntia, dead grass.

Human influence. No obvious disturbance. Mice present, cat scats; didn’t see rabbit droppings. No small birds.

Comment. A mogoplistine and ptinid site; it also has a new species of thysanuran and an abundance of Loxosceles spiders.
Horse Point Plain (HPP10) GPS 15 56.013 S, 05 39.654 W

Creeper plain on the main plateau of Horse Point. Altitude 400 m. A flat and stable open plain with compacted dust and small rocks, both embedded and on the surface.



Vegetation. 80-90% cover. Plant ranking: 1 Carpobrotus (almost pure); 2 Atriplex. Rather few good retreats for invertebrates under rocks, but the creeper (Carpobrotus) plants provide good microhabitats.

Human influence. Disturbance not serious, though a good track passes close by. One mouse in trap and one cheese gone.
Government Garage Flat (GGF11) GPS (est.) 15 56.76 S, 05 39.48 W

A substantial and almost level plain of dust and gravel close to the rim of Fishers Valley south of Holdfast Tom. Altitude 300-310 m. Aspect due east. This site is the closest analogue of the dusty sites in the Central Basin, but has not been well characterised.



Vegetation (rough estimate from photo). 50% cover. Plant ranking: 1 Suaeda; 2 probably Atriplex; 3 prob Mesembryanthemum. Perhaps also some other chenopods. In the surrounding rockier area there is much Carpobrotus and a few Opuntia, but we did not collect there. Lots of ant nests under stones.

Human influence. No obvious recent human effects. Mice may be absent..
Widow Slope (WS12) GPS 15 57.326 S, 05 39.246 W

A stony slope that forms part of the southeast wall of the Central Basin of PBP. Altitude 310-330 m. The slope faces NNW and is about 25o, moderately stable but eroding, with rocks of varied size over fine dust.



Vegetation. c.50% cover. Plant ranking: =1 Suaeda, Atriplex; 3 Mesembryanthemum. Also present: Carpobrotus, Hydrodea, Opuntia, grass, Portulaca.

Human influence. Some old wheel tracks up slope. One mouse caught.

Signal House Ravine (SHR13) GPS (est.) 15 56.79 S, 05 38.83 W

Very steep slope below King and Queen Rocks and the Signal Station. Altitude 270-300 m. This is near the head of the steep unstable gully (called Signal House Gut by Shelco) that joins Fishers Valley canyon near Prosperous Bay. The site is very exposed but faces west and is thus sheltered from the prevailing winds and probably receives little condensation. It occupies a worn away place between two dykes, sloping at ~35o and is very unstable and friable, making access awkward. The site is effectively an exposed section through varied volcanic strata, including dykes, burnt soil, aa lava and ignimbrite. The substrate is stones and rubble and mud, with mud slides in places.



Vegetation. c.15% cover. Plant ranking: 1 Suaeda, which is by far the most important; 2 Euphorbia heleniana. Also present: one Hydrodea, one thistle and a few tufts of dead grass. No Atriplex. Nearby there is Nicotiana.

Human influence. A narrow footpath runs along the lower boundary but there is no human disturbance within the site. Mice may be absent.

Comment. Animals are hard to find; there are some on the Suaeda and under rocks, and mogoplistines were found in a hole in a steep dusty bank. This is the site where we rediscovered the “fossil” gastropod, Nesopupa turtoni.
Oil Drum Creeper (ODC14) GPS 15 57.067 S, 05 39.389 W

A featureless and exposed plain in the PBP Central Basin, around a discarded oil drum. Altitude 295 m. Contrasts with nearby Samphire Plain in the presence of lots of light whitish rocks on the surface and a stony rather than dusty substrate. There are plenty of retreats for animals since the rocks are generally not well embedded and there are spaces below that seem good for animals such as Xeropigo spiders.



Vegetation. Around 75% cover. Plant ranking: 1 Carpobrotus, which is almost the only higher plant. Suaeda is present in surrounding areas and there is a rich lichen flora on the rocks.

Human influence. Old disturbance for aerials etc. However, now most of the trucks etc go along the main track, so the area is not driven over. There are donkey droppings and a feral cat was seen at night. No information about mice.

Comment. No lycosids seen at night, and no dictynids were found. Brown Widows Latrodectus not recorded though there may have been one in the oil drum.
Stony Outwash (SOW15) GPS 15 57.147 S, 05 39.562 W

Sloping outwash fan at the west end of the PBP Central Basin, originating at Upper Bone Gully (PBP5). Altitude 310-330 m, and with northeast aspect, so the site is exposed to the prevailing winds. The slope of the fan is very gentle (c.5o), with pebbly areas interspersed with raised sandy hummocks. Trapping was in the pebbly areas. The substrate is a mixture of sand and pebbles, with some parts harder to dig into than others, because there are pebbles underneath in some places, or hard compacted dust. It does not smell of guano. Only very occasional exposed rocks, but some of these form refuges for Latrodectus; much of the searching was under small stones. Fossil bones of the St Helena Hoopoe were found here in 1995. Wirebirds seen.



Vegetation. c.20% cover. Plant ranking: 1 Suaeda; 2 Chenopodium. Other plants present: Carpobrotus, Mesembryanthemum, Hydrodea, Opuntia and a grass.

Human influence. No significant recent disturbance, though old concrete aerial bases are present. No evidence about mice.

Comment. Far more animals under the small stones than would be expected. These include pseudoscorpions (only found at one other site) and beetles, mainly Anthicodes, which must have very special adaptations to withstand the heat under the thin rocks. Ants are not easy to find but are abundant in traps, so are probably nocturnal.
Horse Point Dump (HPD16) GPS (est.) 15 56.10 S, 05 40.04 W

Northwest edge of dusty plateau with northwest aspect, thus sheltered from prevailing winds. Searched only at night.


Plateau Trig Point (PTP17) GPS 15 57.382 S, 05 39.127 W

Plateau forming part of the southeast rim of PBP where it is interrupted by the gully leading towards Gill Point. Altitude 325 m, extremely exposed, but stable. Flat or slight slope to northeast, Angular broken stone on surface in patches interspersed with gravel and dust; exposed bedrock in places. Hardly any prominent rocks.



Vegetation. Cover not recorded but around 30%. Plant ranking: 1 Suaeda, very prostrate; 2 Atriplex; =3 Carpobrotus, Hydrodea. Also present: Chenopodium, Grass, various lichens. Collecting was in areas away from Carpobrotus.

Human influence. Extensive rock collection in the past. Mice may be absent: none caught and cheese not taken from nails.
Prosperous Bay Hinterland (PBH18) GPS 15 56.161 S, 05 39.068 W

Final detritus fan of Fishers Valley watercourse, immediately behind the stony beach at Prosperous Bay, with some traps among beach rocks probably thrown up by waves. Altitude <5 m. Nearly flat and stable except in flash floods, but gullied by water flow. Faces north and largely sheltered from prevailing east wind. Substrate comprises rocks, gravel and silt washed down watercourse, and includes some big rocks. Very dry even in the watercourse. Not easy to find animals, and some traps were lost.



Vegetation. Around 50% cover. Plant ranking: 1 Suaeda; 2 Pennisetum macrourum (Thatching Grass). Also present: Nicotiana.

Human influence. Frequent human activity: fishermen, sometimes with donkeys, and casual visitors. Picnic remains and evidence of past burning of grass and Suaeda. Rabbit droppings. No evidence about mice.

Comment. This site had the new thysanuran, which otherwise occurred only at Bryan’s Rock.
Prosperous Bay Gully (PBG19) GPS 15 56.382 S, 05 38.971 W

Bed of gully around half a kilometre inland from Prosperous Bay, in Signal House Gut. Altitude 30-50 m, gentle slope around 5o, stable except in floods. Faces northwest, very sheltered. Large boulders and smaller rocks and grit in gully, with some fine dust.



Vegetation. c.50% cover. Plant ranking: 1: Suaeda; 2 Nicotiana; perhaps some Carpobrotus.

Human influence. Disturbance negligible. Rabbit droppings. Mice probably present.
Stone Hill Springs (SHS20) GPS (est.) 15 57.11 S, 05 39.17 W

Gravelly plain or shallow outwash where moisture was first noted at night and interpreted as seepage; but perhaps it is condensation. Adjacent to PBP2 and species records now included with that. Altitude 295-305 m. There are lurkers Trochosippa sp. but not Hogna nefasta.


Strange Rock Plateau (SRP21) GPS 15 57.152 S, 05 38.992 W

High stony plateau on eastern rim of PBP. A large flat area that is extremely exposed to prevailing winds. Altitude 315-325 m. A gravelly and almost level surface with much dust in the substrate. Bedrock seems to be black lava, but surface gravel includes much of the strange white rock, with yellow ochre coloured dust. But this layer is shallow, so it is very hard to get stakes in or to excavate for traps. Where excavated the rock is white. Ian Baker thinks that this whole area was originally much higher to match the surviving mesas to the north, but the height diff is only ~12 m. The basalt(sic) at the surface here has probably disintegrated under action of sun and then turned to dust and gradually blown away, always leaving a hard pebbly surface.



Vegetation. 10-20% cover, but very patchy. Plant ranking: =1 Suaeda (prostrate), grass; 3 Hydrodea. Also present: Carpobrotus.

Human influence. Very little human disturbance except for excavations at edge and some wheel tracks. No evidence about mice.

Comment. No lycosid eyes seen in a careful night search that also included the head of the ravine near the pile of white rock rubble (sample coll.). The surface rocks are almost all <15 cm and mainly <5 cm, giving no retreats for widows and Xeropigo, although one immature of latter was found. But small rocks had lots of animals under them, rather as in SOW15. Thysanura abundant but very patchy: often a whole group together, then none. Anthicodes also common, and several Tarphiophasis leleupi found. Under-rock spiders are mainly oecobiids (2 spp) with dictynids in second place numerically. No salticids were seen in day or night search: they may really be absent or scarce. Primnia grasshoppers did not seem numerous but were present. At night a few Microxylobius westwoodi were found up on biggish twigs of Suaeda, where they seem to scrape away in a microscopic way at the dead twigs, leaving paler marks.
South Basin Samphire SBS22 GPS 15 57.227 S, 05 39.351 W

Level area just south of Stone Hill on either side of track. Altitude 305 m. Substrate of whitish grit, with particles slightly larger than in nearby areas (SP8 and BYD24) with yellowish dust.



Vegetation. 60-70% cover. Plant rankings: 1 Suaeda; 2 Mesembryanthemum; present Carpobrotus, grass, Opuntia.

Human influence. A well used track goes through the site, and there are donkey droppings. No evidence about mice.

Comment. The white grit seems fine for burrowing lycosids, and after rain there were very large numbers of open burrows.
Holdfast Tom Samphire HTS23 GPS 15 56.366 S, 05 39.610 W

The floodplain of the gut draining a broad valley south of Horse Point, southwest of Holdfast Tom and well back from the rim of the precipitous Fishers Valley canyon. Altitude 310-330 m. Slope perhaps 5o, with east aspect. Substrate of deep dust, deeply gullied in places and where stable often with a layer of lichen on the surface.



Vegetation. 40% cover. Plant ranking: 1 Suaeda; present but very scarce Carpobrotus, Mesembryanthemum, Hydrodea, Chenopodium, Opuntia.

Human influence. A walking track to Holdfast Tom goes through the site. Edward caught a mouse.

Basin Yellow Dust (BYD24) GPS 15 57.276 S, 05 39.377 W

Dusty plain in the lowest part of the Central Basin of PBP, close to SP8. Altitude 305-310 m. Almost level but somewhat gullied, and with a substrate of deep and fine yellow dust. Tiny dust devils are common in this part of the basin and much of the dust must be deposited from the air, but occasional flooding is also important. Rocks and pebbles are almost entirely absent.



Vegetation. 30-40% cover. Plant ranking: 1 Suaeda; 2 Chenopodium. Also present: Carpobrotus, Mesembryanthemum, Hydrodea, grass, and the star-shaped dead chenopod.

Human influence. A track goes through the site. Mice seemed abundant.

Appendix 2.


Suggestions for Immediate Action on Prosperous Bay Plain Central Basin, with map information and GPS calibration for marked southern boundary
Submitted by Philip and Myrtle Ashmole

December 2003


Markers to indicate area to be protected. ANRD have put in small metal pipes painted red and yellow at the positions shown on the attached map (Plate 2b), starting with the top left extremity of the solid line near the concrete hut (see GPS readings on next page).

Large notice at entrance points – from Bradleys and also on entrance from Woody Ridge, with wording such as:- “The Central Basin of Prosperous Bay Plain is a fragile environment. Please preserve the native plants and endemic invertebrate animals by keeping vehicles to established tracks.”
Late evening visits by interested people to lamp for spiders. We don’t think that careful walking at night should be too destructive. A good place to go is where we took SHNCG on 8th December, just to the south of Stone Hill.
However, stay away from all stony areas (especially on the slopes surrounding the Central Basin) because the Brown Widow Spider (Latrodectus geometricus) is very common there. At night these spiders come out from under their rocks and hang upside down in very inconspicuous horizontal webs. They are then easily recognisable by the red or orange ‘hour glass’ marking on the underside of the abdomen, which shows up as they hang in the web. The webs are usually strung between low rocks at about 4 inches above the ground, but sometimes between plants. There is thus a real danger of getting a spider on one’s trousers or boots, which could lead to trouble. The Brown Widow is said to be a lot less poisonous than that of the Black Widow, but is certainly best avoided and could be serious for a child. We are now pretty sure that it is only the Brown Widow that is present on PBP. The Belgians have an old record of Black Widow from Half Tree Hollow, but it is possible that they were confused (like us) as some specimens are hard to separate.
Publicity associating the importance of the dusty Central Basin with the Peaks, in relation to both conservation and tourism.
Mark the two main routes across the plain BEFORE all the surveyors and engineers come out to the island.
Draw up conservation protocol to be handed to all these visiting potential contractors.
Halt the advance of invasive species. Remove all cactus and Lantana plants (there are not many yet). Creeper is non-native and an invasive that alters the habitat. The first essential is to remove all creeper outside the main and highly visible patches. If creeper is restricted to these main patches, they can be tackled later if man/woman power exists.
Southern Boundary of Central Basin of Prosperous Bay Plain, as marked with pipes by ANRD staff in December 2003
The metal pipes (painted red and blue) have the following readings on the Environment Department GPS configured as shown below:
1. South lat. 150 57’.049

West long. 050 39’.610

Elevation 335 m

2. South lat. 150 57’.170

West long. 050 39’.658

Elevation 346 m

3. South lat. 150 57’.256

West long. 050 39’.667

Elevation 341 m

4. South lat. 150 57’.264

West long. 050 39’.593

Elevation 337 m

5. South lat. 150 57’.369

West long. 050 39’.425

Elevation 320 m

6. South lat. 150 57’.377

West long. 050 39’.354

Elevation 330 m

7. South lat. 150 57’.397

West long. 050 39’.259

Elevation 331 m

8. South lat. 150 57’.331

West long. 050 39’.160

Elevation 325 m


GPS configuration now set on Environment Department GPS
Map datum settings:
dx -320

dy +550


dz -494

da -251


df -01419270
UTM grid user defined settings should be as follows:
Longitude of origin WO 95 degrees, 0 minutes, 0 seconds

Scale 0.9996

False East: 10 million (or? 9,999,999)

False North : 100,000

Note that when you try to get your position on the GPS screen it may just give you a row of dashes and something like ‘USR’ (user defined). You can get the figures by flipping the top edge of the large central control button.
David Ellis, who provided this information, said “With this calibration you should be able to get within 8 to 10 metres level of accuracy.” But we find that although this is pretty good at the St Helena datum on Ladder Hill, it is about 50 m too far north over at PBP. Some days readings also wander a good deal, and elevations are often unreliable.
David said nothing about magnetic variation, which I think is irrelevant to position finding but important in navigation. I think it must now be about 17 degrees west but any yachtsman can probably give it accurately.

Transfer to OS large scale (1:10,000) map of SH:
1 minute of Latitude is 184 mm on map, ie 1.840 km.

1 minute of Longitude is 178 mm on map, ie 1.780 km.


To enter Latitude south on map multiply last 3 figs by 0.184 and measure top to bottom.

But there is a ~50 m N-S error in the PBP area, with plotted point coming too far north, so add 5mm to the result to get real position on map.


To enter Longitude W on map multiply last three figs by 0.178 and measure right to left.

Transfer to Ashmoles’ personal A4 map:
The scale on this is 1.88 X the OS large scale map.
To enter Latitude south multiply last three figs by (0.184 X 1.88) = 0.346, add 9 mm to result to compensate for error, and measure top to bottom.
To enter Longitude W on map multiply last three figs by 0.178 X 11.88) = 0.335 and measure right to left.




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