Differential GPS 1

[BRIS]

I have an agency funded project to produce water supply and sanitation systems to 7 towns in Yemen (typical population 20,000 to 30,000). There are no existing maps and I have a limited budget. I need to produce maps at 1:2000 scale. I have investigated satellite imagery and I can procure 0.65 m resolution panchromatic imagery at a reasonable price. However processing to produce maps and a DEM is way above my budget. For preparation of water supply and sewerage layouts I do not need high geometric accuracy - I simply need to locate pipes and connections relative to surrounding features. My cad draftsmen can trace the satelite imagery - (the maps will not be geometrically correct but accurate enough).

My idea was to procure the satellite imagery and to use differential GPS to fix points throughout each city. I am more interested in accuracy in level for design of sewer gradients etc than x,y coordinates.

Questions are
1) Is there a better approach - cost and speed rather than accuracy being my main concerns?
2) What basic equipment do we need to conduct a GPS survey - i.e base station and roaming station etc.
3) what is the approx cost of the basic equipment.

Brian

 

[IRstuff]

Since you say you have a "limited" budget, that implies that there is already a line item for this activity. What can you spend, based on the existing budget? Note that you can probably rent the equipment, particularly if this is a limited time frame.

TTFN

 

[BRIS]

Unfortunatly my budget is not in neat lines - I have a line budget of US$150,000 for topgraphic surveys but I have other activities with no budget - my overall budget is inadaquate for the project and I am trying to make savings and reallocations wherever possible. We would procure the equipment rather than hire and hope to spread the purchase costs to future projects.

 

[IRstuff]

I've not done a whole lot of research, but it seems that a DGPS system can be had for ~$20K US:

http://www.navtechgps.com/supply/dl4.asp

The only question then is whether that level of accuracy is required for your application

TTFN

 

[BRIS]

IRstuff - thanks - a complete package was what I was looking for - next is to find a supplier in the Middle East and someone to drive it.

cheers

 

[BRIS]

In pursuing the use of differential GPS I am getting very negative responses from local surveyors. Why is this ?. Surely if GPS will give the required accuracy it must be much quicker and easier to use than total station in congested built up towns.

 

[IRstuff]

But, for an entrenched traditionalist, that may be precisely the wrong answer. Traditional surveying requires some modicum of skill, knowledge and lots of careful measurements, e.g., job security.

Additionally, change can be perceived to be both painful and unnecessary as well as driving some people so far outside their comfort zone as to be anathema.

Furthermore, DGPS, being a wholly independent measurement system, may reveal heretofore undetected or hidden errors in surveyance.

The sell job may be to not push the accuracy and speed, per se, but to possibly appeal to their greed, in that they could charge the same amount, but do more jobs and therefore make more money with the same amount of hours worked. The other selling point is that they can be in on the ground floor and establish sufficient expertise in the new technology to obviate the need for anyone else to learn the technology.

Another selling point is that their ability to find additional or new buried surveying skeletons might be worth something in of itself.

TTFN

 

[Smacin]

What you want is a combination of both DGPS and a Total Station.

Trimble equipment has always been my choice for this situation. As for someone to conduct the work, you could employ experienced staff from your home country to train a new team to complete the work.

Thats what we've done in the past.